When prosecutors believe terrorist suspects have valuable information, and they are willing to cooperate, they will sometimes offer a "snitch visa" to secure their cooperation. The snitch visa program started after the World Trade Center attack in 1993 as a way to get suspects to cooperate. The s-visa usually results in a shorter prison sentence, and allows the suspect a lawful way to obtain citizenship down the road.
As currently implemented, the program only allows up to 50 "s-visas" to be issued every year. If the prosecutor thinks it is a useful bargaining chip, they first tell immigration, and eventually it must be signed off by the attorney general and secretary of state. It is believed that the suspect being held for planning bombings of the New York City subway system has been offered one of these visas to secure his cooperation.