A judge in Chicago has sentenced a federal deputy marshal to four years in prison for leaking information about a witness in a major organized crime case.

John Ambrose was convicted in April in what prosecutors say was the first security violation in the history of the government's Witness Security Program.

Guidelines called for an 18-month sentence at most, but Judge John Grady increased the time on Tuesday, saying he wanted to deter officials from ever leaking secret information again.

The information concerned admitted mob man Nicholas Calabrese, who became the government's star witness in the FBI's landmark Family Secrets mob trial.