A subpoena is an official document commanding a person to appear in court on a specific date and time, or commanding the production of specific documents, subject to a penalty for failure to comply. Oftentimes, after an attorney has reviewed evidence provided in discovery, the attorney may deem it necessary for the issuance of subpoenas in order to obtain additional evidence not previously provided by the police or prosecution, or not otherwise contained in the discovery materials. These discovery materials may include additional police reports, security officers' reports, surveillance video recordings, 911 recordings, and any other evidence that could aid defense counsel in the preparation of the defense of a case.