A federal deputy marshal was sentenced Tuesday to four years in prison for leaking information about the government's star witness at Chicago's biggest organized crime trial in decades.

John T. Ambrose, 42, was convicted in April of stealing government property and violating the secrecy of the Witness Security Program — the network of bodyguards and safehouses used to protect witnesses in mob and terrorism cases.

U.S. District Judge John F. Grady said that the sentencing range under federal guidelines of 12 to 18 months was too low and he wanted to send Ambrose to prison long enough to deter such leaks in the future.

It was the first breach of security in the 25-year history of the Witness Security Program and officials said they believe the program remains safe.

"This is an isolated case based on an isolated event," U.S. Marshal Kim Widup told reporters after the sentencing.

The information was that admitted hit man Nicholas Calabrese was in federal custody and being kept in a safe house in Chicago. Calabrese went on to become the government's star witness in the FBI's landmark Operation Family Secrets case — Chicago's biggest mob trial in decades.

Three big-name reputed mob bosses were sent to federal prison for life and two of their alleged henchmen got long sentences for a murder conspiracy.

Both sides say Ambrose leaked the information to a former Chicago police officer who had gone to prison himself in a corruption case decades ago.

U.S. Attorney Patrick J. Fitzgerald and Robert D. Grant, special agent in charge of the Chicago office of the FBI, both testified that when they confronted Ambrose he said he gave the information to a friend to pass on to the mob in hope he would be repaid with useful information in hunting a fugitive.

Ambrose denies that he ever said he did that. Instead, he claims that he was merely boasting to former Chicago police officer William Guide that he had been given the high-prestige assignment of guarding the only so-called made member of the Chicago mob ever to change sides.

"He shot his mouth off," defense attorney Francis C. Lipuma said. He said he would appeal the conviction.

FBI agents first heard of the leak when they listened to secretly made tapes of two mobsters Michael and James Marcello talking in a prison visiting room. They spoke of a "baby sitter" inside federal law enforcement who was providing information about Calabrese.

The baby sitter was identified as someone who was the son of a former Chicago police officer who had gone to prison in a corruption case years earlier. That quickly led them to Ambrose.

Ambrose remains on unpaid leave with the U.S. Marshal's Service.