Manhattan U.S. Attorney Preet Bharara’s office late Friday filed a letter saying that an FBI agent had confessed to being a “significant source” of leaks to reporters covering an insider trading probe of a noted Las Vegas sports gambler and golfer Phil Mickelson.

The letter was filed after U.S. District Judge P. Kevin Castel ordered a probe into leaks that preceded the indictment of gambler William “Billy” Walters, who now is facing charges. Mickelson never was charged in the case.

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The agent, who was not named, admitted on Dec. 6 to providing information about the probe to the New York Times and the Wall Street Journal, and his case has been referred to the FBI’s Office of Professional Responsibility and the Justice Department’s inspector general for possible discipline, Bharara’s office said.

The letter urged Castel to hold hearings to determine sanctions on the government and the agent, who prosecutors said acted alone. A lawyer for Walters could not be reached for comment, and a spokeswoman for the FBI’s New York office declined to name the agent or to comment.

Walters on June 1 pleaded not guilty to charges of making profits and avoiding losses of $40 million based on tips from an executive at Dean Foods. Mickelson owed money to Walters and received tips but was not charged with knowing they were based on inside information and agreed to repay nearly $1 million.