S visa:

The S visa was created for individuals who either leave the U.S. or fall out of valid nonimmigrant status and agree to cooperate with federal or state law enforcement agencies in the U.S. to aid in the investigation and prosecution of a criminal enterprise, or a terrorist organization or enterprise, and who are deemed by the Attorney General to be critical and reliable witnesses or informants.

An application must be made on behalf of the witness directly by the interested federal or state law enforcement agency, which must be approved at the seat-of-government level, and the witness must sign a statement agreeing to the terms of S visa status.