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CBS 2's Dana Kozlov and Derrick Blakley and the Associated Press contributed to this report.
CHICAGO (CBS) ― A deputy U.S. Marshal was in federal custody Thursday morning for allegedly disclosing confidential, highly sensitive information about an organized crime witness who was under federal protection.

John Thomas Ambrose is charged in a criminal complaint with theft of government property, accused of illegally using his position to obtain information about Nicholas Calabrese while Calabrese was in the U.S. Marshal's Witness Security Program, according to the U.S. Attorney's office. That information was later transmitted to the Chicago Outfit.

Calabrese is expected to be a key witness in the government's Operation Family Secrets murder conspiracy case.

Ambrose, 38, a deputy marshal since January 1998 and a supervisory inspector for the U.S. Marshals Service's Great Lakes Regional Fugitive Task Force, was placed on administrative leave last September. He is scheduled for an initial hearing this afternoon before a U.S. magistrate judge at the Dirksen Federal Building.

Prosecutors said Ambrose told them in a Sept. 6 interview that he passed the information to an associate of reputed mob boss John "No Nose" DiFronzo in hopes of getting information on the whereabouts of then-fugitive organized crime figure Joseph "Joey the Clown" Lombardo.

Gary Shapiro, the first assistant U.S. attorney in Chicago, said Ambrose's action "constitutes an egregious breach of his law enforcement duties."

Calabrese is still in protective custody, set to testify in Operation Family Secrets, the massive case involving several alleged mobsters including Calabrese's brother Frank, Sr.

The feds say despite the leak, Calabrese was never in any danger, but he and others could have been.

"The investigation so far has not uncovered any evidence that either this witness or another was ever in danger and there is no evidence that any attempt was made to harm any protected witness," Shapiro said.

"As a federal prosecutor, this is the sort of case we never hope to see," Shapiro said.

"He is surprised he's been connected to the mob. He's not tied to the mob in any way," said Ambrose's attorney, Francis Lipuma.

Lombardo, among those charged in the Operation Family Secrets indictment, was subsequently captured and is due to stand trial starting in May along with the others accused in the case.

Calabrese, 63, of Chicago and his brother, Frank Calabrese, 68, of Oak Brook are among 14 defendants charged in a sweeping indictment alleging a long-term conspiracy by Chicago mobsters to commit at least 18 murders.

The murders include that of Tony "The Ant" Spilotro, the mob's one-time man in Las Vegas, who was beaten to death and buried in a corn field.

Federal prosecutors say Calabrese has been cooperating with their investigation of Chicago Outfit members and, as an admitted "made" man in the Outfit, he has provided authorities "with the most expansive overview ever of Chicago Outfit murders," including testimony about 16 murders in which he participated and 22 others about which he had second-hand knowledge.

In 2002, he agreed to cooperate in the investigation of alleged Outfit members, including his brother Frank Calabrese, Sr., and brothers James and Michael Marcello. He was formally admitted into witness protection in 2002 and moved to a secure federal prison facility.

The first official disclosure of Calabrese's cooperation in the Outfit murder investigation was in 2005 at a detention hearing for James and Michael Marcello, who were indicted in connection to the murders.

In a number of secretly recorded conversations between the Marcello brothers in 2003, while James was incarcerated in Michigan, authorities learned Michael Marcello had an inside source with information about Nicholas Calabrese's cooperation with the feds, according to an affidavit.

In a recorded conversation in March 2003, Michael Marcello identified his source, whom he called "the Babysitter," as Ambrose, not by name, but by a description that could fit no other law enforcement officer, according to the affidavit.

"We eventually determined the reference to the Babysitter was to James Ambrose," Shapiro said.

The feds also found Ambrose's fingerprints on confidential documents about Nick Calabrese.

The Marcellos allegedly discussed those documents in a recorded conversation.

But Ambrose's lawyer contends the Marcellos' leak may not be his client.

"The Marcellos referred to, for instance, other attorneys, some criminal defense attorneys, guards at the Metropolitan Correctional Center, other law enforcement personnel and even made a reference to the U.S. Attorney office, and an individual in U.S. Attorney's office," Lipuma said.

However, on those tapes, the Marcellos also indicate their sources' father was a member of the Marquette Ten, a group of Chicago cops convicted of corruption in the 1980s.

John Ambrose's father, Thomas, was one of the lead defendants in that case. He died in prison.

The feds charge that John Ambrose was leaking the information on Nick Calabrese to an unnamed "Individual A," another member of the Marquette Ten.

Ambrose lives in a quiet Tinley Park neighborhood. Officials say he was a respected marshal.

If convicted, Ambrose faces up to 10 years in prison and a $250,000 fine.

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