WASHINGTON - Special prosecutor Patrick Fitzgerald took a gamble three years ago that White House press secretary Ari Fleischer might break open his leak investigation.

As Fitzgerald's inquiry was heating up into who revealed CIA operative Valerie Plame Wilson's name to reporters, Fleischer stepped up with an offer: Give me immunity from prosecution and I'll give you information that might help your case.

What prosecutors didn't know was that Fleischer was one of the leakers. Without immunity, he refused to talk. Not even a hint.

Prosecutors normally insist on an informal account of what a witness will say before agreeing to immunity. It's known as a proffer, and Fitzgerald said Thursday that he never got one from Fleischer, who was chief White House spokesman for the first 21/2 years of President Bush's first term.

"I didn't want to give him immunity. I did so reluctantly," Fitzgerald said in court.

Once the deal was struck in February 2004, Fleischer revealed that he had discussed Valerie Wilson with reporters in July 2003, days before leaving his job at the White House.

Fleischer is expected to testify early next week that he learned that the wife of Bush administration critic Joseph Wilson worked for the CIA from defendant I. Lewis "Scooter" Libby, a former top aide to Vice President Cheney.

Libby is being tried in federal court on charges of lying and obstructing Fitzgerald's probe into who outed the CIA operative.

The Fleischer gamble is the second arrangement prosecutors are known to have made with leakers during the probe.

At the onset of the case, former Deputy Secretary of State Richard Armitage said he told authorities that he was the source behind columnist Robert Novak's story that revealed Valerie Wilson's identity and that triggered the investigation.

Fitzgerald has not discussed the arrangement with Armitage but said he granted immunity to Fleischer believing only that he had "relevant information."

The deal Fitzgerald made was unusual enough that Libby's attorneys questioned whether it could be true. They suggested that Fitzgerald got a secret summary of Fleischer's testimony - a deal they want to discuss with jurors when Fleischer takes the stand against Libby on Monday.

Defense attorneys said they would ask U.S. District Judge Reggie B. Walton to force Fitzgerald to reveal what Fleischer promised him. Fitzgerald told Walton that no promises were made.

"We got no specifics," he said.

See Libby trial documents via http://go.philly.com/libby