Deputy U.S. Marshal John Ambrose was sentenced to 4 years in prison Tuesday for leaking information about the star witness in the most significant mob prosecution in Chicago history.

Ambrose told a family friend that he was guarding Nick Calabrese, a mob hitman turned government informant. That information eventually made its way to mob leaders.

The leak was considered one of the most significant breaches of security of the federal Witness Protection Program in its history.

More to come.