A witness who possesses information going to some fact that affects the merits of the case. Generally, the information the material witness possesses has strong probative value and few, if any, other witnesses possess the same information. Because of the importance of a material witnesses’ testimony, judges usually make every reasonable effort to have such witnesses made available to testify. For example, a continuance may be granted in the event that a material witness is unavailable.
Under Section 3144 of the U.S. Code, a material witness may be detained when an affidavit demonstrates that the testimony of the witness is material and that “it may become impracticable to secure the presence of the person by subpoena.” For example, in United States v. Matus-Zayas, Matus–Zayas was charged with illegally transporting and harboring aliens. Five of the aliens being transported were determined to be material witnesses for the case. The court held that it would become impracticable to secure the presence of the material witnesses by subpoena if they were released to Mexico because the witnesses “were citizens of another country and that the [United States] government's subpoena power would be basically ineffectual [in Mexico].”
[Last updated in June of 2020 by the Wex Definitions Team]