Exculpatory evidence

'Exculpatory evidence is evidence favorable to the defendant in a criminal trial, which clears or tends to clear the defendant of guilt. It is the opposite of inculpatory evidence, which tends to prove guilt.

Police or prosecutors are not required to disclose to the defendant any exculpatory evidence they possess before the defendant makes a plea (guilty or not guilty). Per the 1963 Brady v. Maryland decision, prosecutors have a duty to disclose exculpatory evidence even if not requested. Though it is true that the prosecution is not required to search for exculpatory evidence and must only disclose the evidence it has in its possession, custody or control, the prosecution's duty to disclose includes all information known to any member of its team, e.g., police, investigators, crime lab, etc.

In Brady, the United States Supreme Court held that such a requirement follows from constitutional due process and is consistent with the prosecutor's duty to seek justice.

Example
A victim is murdered by stabbing and an accused person is arrested for the murder. Evidence includes a knife covered with blood near the victim and the accused found covered in blood at the murder scene by the police. During the investigation, the police interview a witness claiming to have watched the stabbing occur. The witness makes a statement to the police claiming the stabbing was by another unknown person, not the accused.

The witness's statement is exculpatory evidence, since it could introduce reasonable doubt as to the guilt of the accused. The police believe the witness's account is not true or the witness is unreliable and choose to not follow up on the lead. The prosecutor is obliged to inform the accused and their attorney of the witness statement even if the police doubt the witness's version of events. If they fail to do so, the defendant would have grounds for appeal or for a motion to dismiss.