Criminal trials are often won or lost based on the credibility of cooperating government witnesses. Frequently derided as snitches, rats or turncoats, cooperating witnesses usually carry serious baggage but also provide critical and damning testimony about how the crime was committed.

Cooperating witnesses testify pursuant to an agreement with the government. Pursuant to such agreement, the cooperating witness agrees to plead guilty to his or her criminal conduct, provide what the government is satisfied is full and truthful information about the witness's criminal conduct and the charged offenses, and agrees to be available to the government for testimony at trial. In exchange and assuming the cooperating witness adheres to the terms of the cooperation agreement, the government agrees to bring the witness's cooperation to the attention of the court at sentencing and to move the court for a downward departure from the sentencing guideline range based on such cooperation.

Cooperation agreements long have offered defense lawyers a potentially powerful avenue of attack. Underlying the agreement, of course, is the witness's admission to committing a crime, likely a strike to his or her credibility in the eyes of many jurors. Then there is the cooperating witness's hope of avoiding a long prison sentence through his or her cooperation, arguably a powerful incentive to shade the truth.