Associated Press - April 12, 2009 1:34 PM ET

CHICAGO (AP) - Deputy U.S. marshal John T. Ambrose goes on trial in Chicago based on information from a bugged conversation at the visitors room of a federal prison in Milan, Mich.

Ambrose once was known as a tireless bloodhound who tracked down fugitive gang leaders. The 50-year-old goes on trial Monday.

Federal prosecutors say he told organized crime figures seven years ago a member of the Chicago mob had switched sides and was providing detailed information to federal prosecutors.

Ambrose is accused of leaking information about ex-hit man Nicholas Calabrese, who was the government's star witness at the landmark 2007 "Family Secrets" trial.

Ambrose denies breaking the law in handling secret information.

Read background deatails from our CBS Chicago affiliate on the FBI investigation leading up to the trial.