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“It has a chilling effect from the highest levels to the lowest levels of the CIA,” Mr. Addicott said. “It discourages cooperation from our allies on the war on terror because it strikes at the …issue of trustworthiness. They dont trust us. We have to remember we are in a state of war. These arent peacetime leaks but wartime leaks that have a profound negative impact on our war effort. This contributes to the enemys propaganda agenda.”

However, one intelligence official downplayed the significance of the leak and of the request for a Justice Department investigation.

“These leaks, unlike others in the past, didnt cost the country a viable collection or counterterrorism capability,” the official said. “There were different concepts considered and tested over the years, but they always ran into problems.They never proved themselves, so its not a big loss.”

The official added, “Leaks of classified information are, unfortunately, fairly common.They can do tremendous damage, and they need to be pursued.The real impact here, though, was not operational - these small efforts never took a single terrorist off the street.But it did make for some sloppy stories about hit squads and another public discussion of congressional oversight.”

Because the program was briefed to Congress, it opens the prospect that the FBI could place lawmakers and congressional staffers under polygraph.

The last time the bureau conducted such a major leak investigation involving a member of Congress, it resulted in the reprimand of Sen. Richard C. Shelby, Republican from Alabama, who was accused of divulging to Fox News al Qaeda communications that were intercepted prior to the Sept. 11, 2001 attacks.

At the time, Mr. Shelby was vice chairman of the Senate Select Committee on Intelligence. Sen. Patrick J. Leahy, Democrat from Vermont, resigned from that committee in 1986 after leaking a staff report to a reporter on the Iran-Contra affair, under which the U.S. sold weapons to Iran and used the proceeds illegally to fund anti-communist guerrillas in Central America.

It remains to be seen whether the investigation will rise to the level of those incidents or the 2003 probe that followed the disclosure by columnist Robert Novak that Valerie Plame was a CIA officer. That investigation mired the Bush presidency in legal and political challenges and led ultimately to the indictment of I. Lewis Libby, chief of staff to then-Vice President Cheney. Mr. Libby was found guilty of obstructing the FBI’s original leak investigation.

“Unlike the Valerie Plame matter, where the cocktail circuit knew she worked for the CIA, these people … Blackwater, were covert,” said Victoria Toensing, a former chief counsel to the Senate intelligence committe. “Every fact that I know points to a violation unlike the Valerie Plame matter. The identifier, the exposer, has to know the relationship is covert.”