The government has a duty to preserve certain types of evidence it collects during criminal investigations and prosecutions. This duty exists in order to protect a defendant's rights to due process and a fair trial under the Sixth and 14th Amendments to the U.S. Constitution. The duty relates to the requirement that the government disclose evidence it will use against the defendant at trial, as well as any evidence that is favorable to the defendant. The duty to preserve evidence begins once any state agency or actor has gathered and taken possession of evidence as part of a criminal investigation.
Law enforcement doesn't need to preserve all evidence it collects. The duty to preserve extends only to evidence that might be expected to play a significant role in the suspect's defense: "material" and "exculpatory" evidence. Material evidence is important evidence that's directly relevant to an issue in the defendant's case. Exculpatory evidence is evidence favorable to the defendant in that it clears or tends to clear him of guilt.
Exactly what evidence is material and exculpatory depends upon the circumstances of the case. But alibi evidence is virtually always material and exculpatory; it includes witness statements that place the defendant somewhere other than the scene of the crime and forensic evidence (like DNA) that tends to show that the defendant couldn't have committed the crime.
In many cases, law enforcement doesn't have obvious alibi evidence, but it does have other kinds of material and exculpatory evidence. An example is evidence that impeaches the prosecution's witnesses or otherwise weakens its case. Such evidence may include:
While the police typically collect most evidence used in a criminal case, they aren't the only government actors who have to preserve evidence. The duty also extends to:
The duty to preserve evidence doesn't extend to private people or agencies unless they have a formal relationship with a law enforcement agency. Thus, if a private DNA lab is hired by the prosecutor to examine blood samples collected from a crime scene, that lab must protect and preserve the evidence and the results of any testing it conducts on the evidence.
Defendants have the burden of proving that the state violated its duty to preserve evidence, and that that violation compromised the rights to due process and a fair trial. Courts won't do anything about an alleged violation unless the defendant proves that the evidence in question was material and potentially exculpatory and that the government acted in bad faith. In practice, it's very difficult to prove these points.
In order to prove that the evidence was material, the defendant must establish that
Courts can sometimes infer materiality from law enforcement's actions. For example, the fact that the state normally preserves the type of evidence that it destroyed in the defendant's case may show that the evidence was material. Similarly, the government testing, using, or intending to test or use the evidence provides a strong indication that it recognized the importance of that evidence.
Proving bad faith is tough. It's not enough that government actors were careless or negligent with the evidence—the defendant must show willful, deceitful, or malicious intent. The government failing to follow standard procedures when it lost or destroyed evidence can support an inference of bad faith. An example of bad faith is an officer throwing away a fingerprint sample showing that someone other than the defendant committed the crime.
There are several possible remedies for defendants who learn during trial that the state violated the duty to preserve evidence. They can ask the court to suppress related evidence, exclude or limit testimony about the missing evidence, or dismiss the case. If the missing evidence doesn't surface until after a conviction, overturning the conviction and obtaining a new trial on appeal are possible.