Forensic Evidence

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How accurate is forensic analysis? Learn more about the reliability of each type of forensic analysis.

Gun.jpgFirearm evidence     
Hair.jpgHair and fiber     
Car.jpgPattern and impression      
Bullet.jpgBullet lead composition 

original sources The Washington Post, 16 April 2012


The history of forensic science dates back to as early as the 7th century, when China used fingerprints to identify documents and clay sculptures. Subsequently, in 1248 A.D. a groundbreaking Chinese book, Xi Yuan Ji Lu (洗冤集錄, translated into English as the Washing Away of Wrongs), described various physical differences between the bodies of people who died from natural or unnatural causes, specifically, how to tell the difference between a person who has drowned and a person who has been strangled; it is the first recorded application of medical knowledge as a legal tool to solve crimes. The book represented a significant advance in the field of forensic science, and offers advice that is still useful today.

A criminal defender can be confronted with many different types of evidence when defending a case: sometimes the evidence comes from people: incriminating statements/ confession from the client, statements by witnesses, victims, etc. You may also be confronted with physical (or material) evidence: tangible evidence (as a weapon, document, or visible injury), as well as the results of scientific tests or evaluations performed on that physical evidence. It is this type of scientific evidence that is the focus of this discussion. In many criminal cases, science and technology can be useful in determining the facts and, subsequently, the truth about what occurred. The result of scientific research, tests and analysis can often be the difference between acquittal and a conviction in a court of law.

Forensics is the use of science and technology to investigate and establish facts in a court of law. In preparing for trial, defense counsel must investigate not only the statements of witnesses, documentary evidence and the statements of his client, but the presence or absence of physical evidence. Any physical evidence must then be evaluated to determine its relevance to the case. Physical evidence may be anything tangible: evidence so small that a microscope is needed to see it, or as large as a truck. It may be as subtle as traces of gas at a suspected arson or as obvious as a pool of blood at a homicide scene. There is an enormous range of potential material which qualifies: hair, fibers, blood, fingerprints, seminal and body fluid stains, alcohol, drugs, paint, glass, soil, flammables, tool markings, weapons and foot prints.

Forensic science is merely the application of a broad spectrum of sciences to answer questions of interest to the legal system. It is a multidisciplinary subject, drawing principally from chemistry and biology, but also from physics, geology, psychology, social science, etc.

Forensic evidence can be strong evidence of your client’s guilt or innocence in a criminal case, often the most powerful evidence of all. After all, victims and witnesses can be mistaken, or lying. They may have a bias against you client and want to see him harmed. And we know confessions can be obtained even from the innocent. Innocent clients can admit guilt as a result of coercion, confusion, or fear. Thus, forensic evidence can often be a more objective and reliable way of determining the facts of a case. When properly obtained and examined, physical evidence can answer such questions as "What happened?", “What was the cause of death?”, “What object may have caused the injury?”, and “Who was present at the crime scene?”

Basic Principles of Forensics

There are a few scientific concepts every defender must know about in order to begin to understand, effectively utilize and challenge forensic evidence. These ideas should be considered and reviewed as you evaluate the evidence against your client; they can be valuable tools in putting forth a strong defense, as will be shown in the information and Case Histories that follow.

Law of Exchange

The Law of Exchange principle forms the basis of much of forensic science. It was first proposed in the early 20th Century by Edmond Locard, who founded the Institute of Criminalistics at the University of Lyon in France. Essentially, the law is this:

When two objects come into contact, there is always an exchange

What this means is that every time an individual comes in contact with a place or another individual, something of that individual is left behind at the place, and something of that place is taken away with the individual. Every contact leaves a trace. So when you shake hands with someone, or hug your relative, or sit down on a chair, each contact will leave some piece, or trace, of evidence. This evidence could be fibers from clothes, hair that has fallen out, carpet fibers from shoes, dirt from the ground, skin flakes that fall off the body, etc.

Transfer/ Trace Evidence

This is the material that is left behind whenever there is a contact between two things. It can be a tiny particle of the object, or a hair, fingerprint, blood, etc.

Primary (Original) Transfer

This is the transfer of evidence that would have occurred during the crime itself, for example when the person who committed the crime left behind a fingerprint, or piece of hair. Often evidence used against our clients is alleged to be a primary transfer, and therefore proof of our clients guilt - because it associates them with particular locations, items of evidence or victims. In fact, often such evidence is the result of secondary transfer.

Secondary Transfer

The concept of secondary transfer refers to an exchange of evidence between objects or persons that occurs either before or after the primary transfer of evidence, and that is not necessarily associated with the commission of the crime itself. In some cases, defense counsel may successfully be able to argue that the physical evidence upon which the prosecution is relying in order to establish the client’s participation in the crime is evidence only that the client came into contact with the scene of the crime, evidence or victim at some unrelated point.

Examples of Primary vs. Secondary Transfers

  1. Late at night during a melee in which many people were involved, a criminal stabs a man and drops the knife. He leaves his fingerprints behind on the knife (primary transfer).
  2. After the stabbing, our client, who was also involved in the melee but not the stabbing, picks up the dropped knife. He leaves his fingerprints behind (secondary transfer).
  3. In beating a woman, a criminal has her blood on his hands from the assault (primary).
  4. After the assault, our client approaches the victim in an attempt to help her; he touches her body and as a result has the victim’s blood on his hands (secondary).

Chain of Evidence/ Custody

Chain of custody refers to the sequence of events of a piece of evidence as it goes from the place where it is initially found (scene of the crime) up until the time it appears in court. Think of this as an actual chain, with pieces that are connected. Every part of the chain should be documented - from discovery at the crime scene, through evidence gathering, storage, lab analysis, return to storage, transfer to court, etc. The purpose of such documentation is to ensure that the evidence offered by the prosecution is the same thing the investigators seized, and that it has been in possession of, or secured by, a responsible person at all times. By thus evaluating the chain of custody, one can make certain of its integrity. Every link should be documented by date, time, person handling the evidence, what was done with the evidence by that individual, what tests were performed and where. In many cases there will be gaps in the chain of custody; if the evidence cannot be accounted for and documented in any one step of its journey from crime scene to court room you should argue the evidence has been contaminated and should not be relied on by the court. In so arguing, defense counsel should cite the Criminal Procedure Law which states that evidence must be reliable and sufficient (art. 162(1)) as well as relevant regulations that require chain of custody procedures to be followed in the collection and maintenance of evidence (see e.g., art. 1 of the Regulation of Forensic Science DNA Laboratory GA/T383-2002, “The object that will be tested [by the DNA laboratory] should be packaged separately, and there must be clear indication on the package of name, origin, amount, the date of collection, and the name of the people who collected it”)


Contamination occurs whenever an outside force has an effect on a crime scene or the evidence found within it. When physical evidence has been altered prior to or during its collection and examination then contamination has occurred. For example, when police officers or witnesses enter a crime scene there is always a disruption of evidence. Other factors can include weather, time, decomposition of the body, insects, animals, etc. Potential contamination of physical evidence can also occur at the crime scene, during the packaging, collection and transportation of the evidence, and during evidence analysis and storage. Crime scene contamination results through the actions of the personnel at the crime scene. In general, the greater the number of personnel at the scene, the more likely it is that the scene/evidence will be severely contaminated. Public security officials or other investigators or individuals can deposit hairs, fibers or trace material from their clothing or destroy latent footwear or fingerprints. Footwear patterns can also be deposited by crime scene officers or anyone entering the scene. As the Law of Exchange has taught us, when two objects come in contact with each other they exchange trace evidence. Each time someone enters a crime scene, they not only leave trace evidence behind, but also take evidence away from the scene. Even if a reliable chain of evidence may be established by the prosecution at trial, physical evidence may have been altered prior to or during its collection and examination. A criminal defender should understand that there is usually some contamination of every crime scene; this fact should be argued whenever possible, and used to the clients’ advantage.

Investigation and Evaluation Procedures

The following are a description of select procedures that can be utilized to investigate, evaluate and test physical evidence for use in criminal cases.

Scene of Crime

Much of the physical evidence that is ultimately utilized in court is taken from the scene of the crime. While this information can be invaluable in ascertaining the truth in a criminal case, certain procedures must be followed in order to ensure the utility and integrity of this evidence. The validity of forensic evidence is only as good as the manner in which it was (1) collected, (2) preserved and transported, and (3) analyzed. It is critical that when crime scene evidence is presented, a defender investigate and challenge the way the evidence was obtained and tested. In major crimes like a homicide, investigators must gather evidence to determine:

  1. Who killed the victim?
  2. What exactly happened?
  3. When did it happen?
  4. How did it happen?
  5. Did it happen here or was another crime scene involved?
  6. Who is the victim?
  7. Why was this crime committed?
  8. What evidence is there to help prove the motive and the crime?

In a homicide case, the crime scene can provide significant information on which to base an investigation. For example, an investigation of the scene may reveal the identity of the victim, the approximate time of death, and important evidence and/ or clues to the circumstances of the death. Anything and everything may constitute evidence at the crime scene.

The Collection of Evidence

Many times investigators will not take the proper precautions needed to insure the validity of forensic evidence. Remember the concept of contamination: Investigators who are at the crime scene can deposit hairs, fibers or trace material from their clothing or destroy latent footwear or fingerprints. Footwear patterns can also be deposited by crime scene personnel or anyone entering the scene. As the Law of Exchange tells us, when two objects come in contact with each other they exchange trace evidence. Each time someone enters a crime scene, they not only potentially leave trace evidence behind, but also take evidence away. Further, investigators may neglect to collect evidence that is essential to determine what actually occurred. That is why it is important to find out what steps investigators did (and did not do) in finding material evidence. The following is a detailed list of what investigators should do at a crime scene. If you discover that some of these steps were not taken, there is a possibility that evidence may have been neglected by investigators, contaminated, or otherwise distorted. If investigators do not adhere to best practices when investigating the scene of a crime you may argue the evidence gathered is unreliable and therefore should not be accepted by the court. On the other hand, if the prosecution does not introduce physical evidence that one would expect to find at the scene of the crime (examples: no blood where it should have been; no fingerprints or the presence of unknown fingerprints at the scene; fruitless searches of the suspect’s person, house or car), defense counsel may argue that the police thoroughly investigated and analyzed the crime: Physical evidence was not discovered as it did not exist. Other evidence such as witness statements or the client’s confession are unreliable as witnesses, or even the client, were mistaken, inaccurate or lied about what occurred. The client is innocent of the charges.

The Autopsy

An autopsy is the examination of a body after death. The autopsy is performed by a forensic pathologist and reveals aspects such as its cause, the weapon/s used (if any are involved) and the time since death. Evidence found on the body can be used to deduce the cause, time and manner of death, the key issues in any homicide case. The cause of death refers to why death occurred (e.g. due to excessive loss of blood) and shouldn't be confused with how the victim was killed i.e. the manner of death. A variety of measures are taken by pathologists to establish whether the manner of death was accidental, natural, suicide or homicide, depending on the situation and case type. Forensic science provides a number of solutions to solving the mystery question of when a person died. Generally, the longer it has been since the death, the less accurate the estimation given by forensic pathologists. Forensic pathologists provide valuable information about the manner of death after the autopsy has been performed. There are four main categories regarding the manner of death. These include natural deaths, accidental deaths, homicidal deaths and suicidal deaths. The information found during the autopsy not only reveals the causes of death, how, when and where they occurred, but also shows the manner of death.

Natural and Accidental Death: Natural deaths are the cause of the majority of deaths that occur. The manners of death in this category include heart failure, disease, and death during sleep etc. The autopsy reveals certain aspects of the death, whether it occurred suddenly, unexpectedly or if the person was critically ill. Accidental deaths are common but if public security officials or prosecutors suspect that the accident was deliberate or could have been avoided, a criminal investigation may be conducted.

Suicide and Homicide: Suicide means intentionally taking one's own life. Homicide is the killing of a human by another. Some common examples of homicide include stabbing, smothering, strangling, hitting with a blunt object and burning. Each weapon leaves distinct marks on the corpse, which a pathologist may identify belongs to a certain type of weapon, thus leading investigators to the actual tool used to commit the homicide. It is important to note that a forensic pathologist cannot make assumptions about the intent of the perpetrator or whether a specific crime was committed. They can only provide scientific evidence which will help a court determine if a crime occurred.

Specific Physical Evidence to Consider in Homicide Cases

If the victim’s death was a homicide, there will always be marks of violence on his/her body, no matter how hard the perpetrator may have tried to hide it. During an autopsy, these marks may be difficult to find if the murder agent was drugs or poison, but these agents can still be found through blood tests. On the other end of the scale, signs that the victim suffered a violent death should be immediately discovered from an external examination of the body.

Examples of Common Wounds


Bruising on the skin occurs when the blood vessels are broken by some form of hard and forceful contact with the skin, usually by a blunt object. The shape of the bruise can often reveal which direction the blow was received from and the color of the bruise can indicate generally how long ago the injury occurred. As bruising heals, it goes red-purple, to brown, to green and finally to yellow. Bruising is not an accurate way of deciding how the victim died, as interpreting bruising is different in every person, due to the fact that people bruise at different rates and bruising continues for a short while after death. Strangulation around the neck also leaves significant bruising. The hands, cords and ropes usually leave a distinct mark around the neck in the shape of the pattern on the strangling agent. If the strangling agent is very soft material, it may leave little or no marks, but a dissection of the neck area is able to show tissue bruising beneath the skin.


The shape of a cut in the skin can show whether the weapon had one or two cutting edges, while the angle and direction of the cut can reveal whether a death may have been accidental or intentional. Also, the deepness of the wound can show how much force was used during a stabbing and may also expose for the court whether the criminal intended to kill his victim. Cuts present on the hands can reveal if there was a struggle with a knife, meaning that the perpetrator who committed the crime could also be wounded. Lacerations on the skin can also provide more information on the type of weapon used, though it is often inaccurate when trying to find out the width of the blade as the weapon may have been moved after the original cut was made.

Internal Wounds

Assault leaves signs such as ruptures, internal bleeding and broken bones. During an assault, the abdominal organs are most easily damaged, as the body offers no protection for these organs, unlike the heart and lungs, which are protected by the rib cage. The victim usually dies from internal bleeding into the abdominal cavity rather than organ failure. Broken bones occur most commonly in the nose, jaw and ribs as these bones are more fragile, than, for example the bones in the legs and arms. Broken bones may appear in both the left and right side of the body, although they are more common on the left, as this is the side that is raised to fight off a right-handed attacker.

Establishing Identity

One of the biggest problems public security officials may face with an investigation is the need to confirm or recognize an unknown identity. This situation is commonly found in homicide cases which involve a skeleton rather than a whole corpse or a body which has decayed beyond recognition. In dealing with these cases, forensic science has developed a variety of techniques including: analysis of personal items, including clothing, found with the body, facial reconstruction in the form of computer or clay reconstruction, hair analysis, DNA matching, dental matching, examination of bones and blood analysis. These procedures may be employed to identify the victim even when the body is badly decomposed. In every case, defense counsel should carefully assess what procedures were or were not utilized to identify a homicide victim. Further, defense counsel should be prepared to point out to the court errors that can occur when investigators rely on witness statements, rather than scientific evidence to determine the identity of the victim. Whenever possible, defense counsel should insist that scientific means be utilized to identify a victim and bring challenges to the prosecution’s evidence when it is likely that human error may have been involved in establishing the identity of the homicide victim.

Testing Physical Evidence

In conjunction with consistent witness accounts, scientific testing may be a powerful instrument of corroboration in a criminal case. In the face of conflicted witness accounts, it may provide an objective view that points to one possibility over another. In the absence of witness accounts, it may be used to investigate and establish the actions that occurred at the scene of a crime. Forensic scientists analyze physical evidence collected at a crime scene or from a victim of violent crime in order to provide objective, scientifically-based information about whether an individual may (or may not) be linked to a crime. Physical evidence, in this sense, means tangible evidence, and may include "trace" evidence (such as hairs, fibers, glass fragments, etc.) or biological evidence (such as blood, semen, saliva). In many cases the evidence associating someone with a crime is circumstantial. In these cases, courts may rely on the results of forensic analyses to help determine guilt or innocence. Examples of useful scientific evidence and testing procedures include DNA testing, drug/ alcohol testing, bloodstain analysis, and material analysis.

Homicide Tools

The weapon used as the homicide tool often holds important evidence as to the motive and identity of the murderer. Further, the unique wounds left by different weapons provide much information on the type, size and make of the tool. Almost anything can be used as a weapon. Examples of Common Weapons:

Blunt and Sharp Edged Objects

Objects without a sharp cutting edge, such as rocks, wooden posts, cooking pots, furniture, hammers and even fists, when used as weapons, will cause blunt trauma. External bruising, fractures and broken bones are strong indicators of blunt trauma and that a blunt object was used in the attack. The type of injury and in particular, the shape of bruising can indicate the kind of weapon used. Objects with a sharp cutting edge, such as knives, axes or scissors, will cause sharp trauma. The type of cut on the body can reveal what type of blade was used. Different types of knives include double-edged, single-edged and serrated edged knives. Double-edged knives have dagger-like cutting edges that leave markings of two sharp edges. Single-edged knives have one sharp edge and often create wounds that have a boat-like shape. Serrated knives often give the wound a rough appearance around the edge.

Weapons of Strangulation

If a victim is killed by strangulation, marks or bruising can be left around the neck. This bruising can indicate whether death was inflicted manually or using an object. The size and markings on the wound reveal whether the actions were repeatedly used and whether the rope or cord was twisted or flatted out around the neck. Any long, flexible and tough object is suitable as a weapon for strangulation, e.g., household items like belts, a telephone cord or a scarf.

Cars as Weapons

Cars can frequently be used as weapons in homicide cases, whether the driver was drunk, on drugs, reckless or intended to kill his/ her victim. Forensic scientists can play a vital role when it comes to collecting and analyzing evidence to determine what happened. Investigators look for evidence to verify how fast the car was going, in what direction the car was moving, and whether the driver tried to brake. All this evidence can help the court to determine whether a crime was committed.

Challenging the Prosecution’s Case

Evaluating the Presence or Absence of Physical and Forensic Evidence

There are many things a criminal defender can and should do to confront the physical and forensic evidence presented against their client. Scientific evidence is only as good as its collection and analysis, thus you should review, evaluate and, in appropriate circumstances, challenge both. Generally, the areas to think about/challenge when you have a forensics case with material evidence involve at least one (usually more) of the following:

  • How was the evidence obtained - - at the crime scene, from your client, from victim? (Was it contaminated)?
  • How was it was handled? (Contamination/ Chain of Custody Issues)
  • How was it was analyzed? (Mistakes in Analysis)
  • How were any scientific conclusions against your client reached? (Basis for Conclusions)
  • re there any alternative theories which explain this evidence that leads to innocence? (Secondary Transfer)
  • What things were not done by the police, scientists, etc. that may support innocence? (Failure to Obtain and/or Analyze Other Evidence, etc.)

The forensic expert Paul Kirk has recognized that physical evidence has its limitations. He ends the statement quoted above concerning the objectivity of physical evidence as follows:

…only its interpretation can err. Only human failure to find it, study it and understand it can diminish its value

The defender must challenge the sufficiency of the prosecution’s case, and in addition question the reliability of what has been offered by the government. That is, you are arguing that there has not been enough evidence presented to establish guilt, and that even if there appears to be a significant amount of evidence, it is not trustworthy enough to be relied on by the court. As was said before, forensic evidence is only as valid as the way in which it was collected and analyzed. Beyond challenging the actions of the investigators and prosecutors, the attorney must also review any expert/scientific opinions used against their client.

Source v. Identity

One important limitation regarding forensic evidence must be kept in mind - while the source of 2 pieces of physical evidence can be matched generally, absolute identification is impossible to determine. What we mean is this: Forensic evidence can determine that hair found at a crime scene is consistent with that of the defendant, but analysis can not state that it is in fact the defendant’s hair. In the same way, analysis can state threads of a blue shirt worn by the defendant are identical in structure and chemical composition to those found at a crime scene, but can not state with certainty the threads at the scene came from that shirt. The most that can be stated is that the threads could have come from the defendant’s shirt. It also, of course, could have come from any other shirt manufactured with similar thread. In the United States there have been over 100 cases where the defendant was sentenced to death but subsequently proven innocent. Many of the convictions were based upon faulty “expert” testimony, including cases where:

  1. “Experts” examined evidence and made conclusions when they were not qualified to do so.
  2. “Experts” using unscientific or faulty methods of analysis.
  3. “Experts” falsified experiments to strengthen the prosecution’s case.

The Absence of Evidence

It is not only the evidence that is found at the scene that is important, but it can also be just as vital to the defense to pay attention to what was NOT found at the scene – either to request further investigation, or to point out to the court at trial. The lack of evidence can indicate the investigation was not done properly, which brings into question the sufficiency of the prosecution’s case, or the lack of certain evidence can be used to contradict witness testimony, to challenge the prosecution’s theory, and to support the client’s version of events.

Missing evidence due to failures to properly collect relevant evidence or do necessary testing

As indicated earlier in these materials, investigators should follow proper procedures in gathering evidence at a scene, including taking photographs, to ensure that all of the relevant evidence is obtained. However, sometimes not all of the relevant evidence is collected, either because the police did not consider it relevant, or it may have just been missed. All areas of bloodstain or spatter at the scene should be documented and tested. Weapons should have been tested for the presence of fingerprints, blood, hair and fibers. Sexual crimes should involve DNA testing of bodily fluids present at the scene or on the victim. Autopsy reports should include photos and detailed descriptions of all wounds and marks on the body, and toxicology tests should be done to determine if any toxic substances, drugs or alcohol had been consumed prior to death. If there is any issue of identity of the body, DNA testing should always be completed. The prosecutor should always be requested to obtain further evidence at the scene or to have testing done on any evidence found if it is obviously relevant to the defense. If the further testing cannot be done for whatever reason, it should be argued at trial that the missing evidence is an unknown which brings the sufficiency of the prosecution’s case into doubt. Never forget that the results of further testing may completely exonerate your client – most of the wrongful convictions in the United States have been overturned because of DNA evidence.

Lack of expected evidence, given the circumstances alleged by the prosecution and witness testimony

With this type of missing evidence, the evidence in the case has been properly gathered and tested. This situation arises when the theory as alleged by the prosecution or the witnesses would lead to the expectation of finding certain evidence that just isn’t there. Here are some examples of where the evidence that is not there is important for the defense:

  • If the prosecution alleges that a murderer was interrupted during a bloody attack on the victim and had to flee, and the weapon was left at the scene with no blood on it, the absence of blood on the weapon brings the prosecution’s case into question. It may be important that the murderer was not interrupted and had time to clean the weapon, or perhaps a different weapon was used.
  • If the prosecution alleges that a stranger broke into the victim’s home through a locked door but there is no damage to the lock, that fact causes problems for the prosecution’s theory. The victim may have known the attacker, and let them in willingly; or the victim may be lying about the entire incident.
  • If a witness claims that a victim was punched in the face, and there is no bruising to the victim, that witness’s credibility can be brought into question.

In some cases, what is not present at the scene may be more important than what is there, from the defense point of view. The defender should always keep in mind, when reviewing the evidence from the scene, what the prosecution’s theory is, what the defense theory is, and how the evidence fits with or contradicts those theories.

The Critical Importance of Discovering and Utilizing Material Evidence in Defense of a Criminal Case

A criminal defender must know all of the material evidence in a case before they can use it to the client’s advantage to confront and challenge the prosecution’s case. It is therefore critical for the defender to utilize relevant law and procedure as well as their own investigation skills to obtain as much information as possible from the prosecution, their client, witnesses, the crime scene, and experts. The information contained below can help get you started. Understand, however, that this discussion is NOT an exhaustive and complete list detailing all the things a defender should do in preparing their case; it only concerns discovering material/forensic evidence. For a more complete discussion of investigative techniques generally please consult other materials and sources provided (Legal Aid Criminal Defender Toolkit, etc.) Many times, good investigation can help turn a case that seems destined for conviction into an acquittal. Further, information gathered during investigation can help convince the court that the client is less culpable or otherwise deserving of leniency from the court. For these reasons, a comprehensive investigation must be performed in every criminal case. Although expert witnesses have commonly been used in civil cases in China, they are now being used more frequently in criminal cases. The following information offers suggestions for the criminal defense lawyer on when to obtain an expert, what types of cases usually require expert assistance, and how to challenge expert evaluations that are suspect. Criminal defenders should initially be aware that Chinese law allows them to review and challenge expert testimony offered by the prosecution. Further, it provides lawyers the right to make an application to the court requesting supplementary or new expert evaluations. Finally, the law allows defense lawyers to conduct an independent investigation into the facts and circumstances of the case.


As we stated at the beginning of this outline, there are many things a lawyer can do to confront forensic evidence used against their client. Indeed, in many instances the lawyer may be able to use such evidence to his advantage. The first step is always to conduct extensive, detailed interviews with your client, to alert you to possible defenses, and the existence of evidence which may help to advance the defense or refute the prosecution’s case. It is also vital to completely understand the prosecution’s case against your client – the witness statements, statements of your client, and any physical evidence and scientific reports. An understanding of investigative techniques and forensic science will help the defender to see where further investigation or testing may be required, and where experts may be of assistance to properly prepare and present a persuasive, consistent defense theory to the court. Expert witnesses can provide valuable assistance to criminal defenders by helping them to understand complex scientific, valuation, authenticity and other highly specialized issues and by assisting them in putting this complex information into a format that is both persuasive and easy to understand. Additionally, experts can help lawyers critically assess the weaknesses in the prosecution’s evidence and create a defense theory that will successfully refute it. Expert witnesses should be regarded as vital members of the defense team in any case where their knowledge, education and experience can be utilized to explain why the prosecution’s version of events is incorrect.

By carefully investigating the case, analyzing the evidence, reviewing any tests, and being creative in thought, a skilled defender can successfully challenge forensic evidence, and often use it to their client’s advantage in court.

See Evidence, Unvalidated or Improper Forensic Science