There are generally two types of evidence in criminal trials in Virginia and around the country. Inculpatory evidence is presented by prosecutors and tends to incriminate defendants and establish their guilt. Exculpatory evidence is presented by criminal defense attorneys to exonerate their clients or raise questions about the validity of inculpatory evidence. Defense attorneys will usually try to gather exculpatory evidence by using investigators or making inquiries of their own, but this kind of material may also be provided by prosecutors.
This is because the U.S. Supreme Court ruled in 1963 in Brady v. Maryland that prosecutors must turn over all impeachment and exculpatory evidence to defense attorneys and defendants. Impeachment evidence is material that could cast doubt on the credibility of witnesses called by prosecution attorneys. This is why prosecutors tell defendants when they have entered into plea agreements with witnesses in return for their testimony. The criminal records of prosecution witnesses are another kind of impeachment evidence.
Examples of exculpatory evidence include witness statements that contradict the prosecutor's version of the events in question and forensic evidence collected at a crime scene that belongs to an unidentified or uncharged third party. Prosecutors who fail to abide by the Brady rule can be sanctioned for professional misconduct and could even face criminal charges and potential jail time.
While the Brady ruling is clear, it allows prosecutors to use discretion when deciding what exculpatory evidence must be turned over because it relates directly to innocence or guilt. This is why experienced criminal defense attorneys generally make independent efforts to gather this material.