Chain of Custody
In a case that involves physical evidence, there may be chain of custody problems that need to be explored, that could mean that a particular piece of evidence is not what it is alleged to be, for example the knife the prosecutor introduces into evidence is not the actual knife that was used. If the prosecution's case rests on the identity of blood found on a knife, it will be important not only to know what the knife was properly used for before the crime, and also whether the knife was properly handled, or properly stored, after taken as evidence in the crime. A bystander, or the victim's family or the co-defendant, seeking favor with the police, may produce any knife or other weapon simply to be of assistance. It could be that the lab that tested the blood did not properly clean the equipment that was used and contaminated the results.
The evidence could be contaminated if it was not handled properly. For example, if a bomb was used to kill the victim, and the police bring bomb detecting dogs or other bomb specialists to the defendant's house to look for evidence, explosives residue from another investigation the dog or specialist investigated may be inadvertently tracked into the defendant's home. It is critical for defense counsel to investigate how evidence was collected at the scene, how it was stored at the police headquarters, and how it was handled by experts to be able to question the reliability of the procedures. This investigation will require counsel to critically think through how the investigation was conducted, visit the scene of the crime, inspect the evidence that was collected, review the standard evidence collection procedures and compare to how the evidence was collected in the individual case, and consult with experts.
See also Evidence