National security adviser Condoleezza Rice yesterday promised Congress a full investigation into allegations that an Iraqi politician supported by the Pentagon told Iran the United States had broken the code it used for secret communications, and U.S. officials said the revelation destroyed an important source of intelligence.

In a closed-door damage assessment on Capitol Hill, National Security Agency officials said the disclosure cut off a significant stream of information about Iran at a time when the United States is worried about the country's nuclear ambitions, its support for terrorist groups and its efforts to exert greater influence over Iraq.

"It's a very important ability, to be able to intercept their communications," said Sen. Dianne Feinstein (D-Calif.), who attended two briefings on the matter yesterday. "A very valuable tool the United States had was taken away."

Iraqi politician Ahmed Chalabi, whose exile group received more than $40 million in U.S. payments over the years, denied yesterday that he disclosed secrets to Iran, and he demanded that the Bush administration investigate the source of the leak about the investigation of him.

In a letter to Attorney General John D. Ashcroft and FBI Director Robert S. Mueller III, Chalabi's attorneys said the leak came from "the same individuals within the U.S. government who have undermined the President's policies in Iraq . . . and are using Dr. Chalabi as a scapegoat for their own failures."

The allegations against Chalabi have hit as controversy grows over his role in helping to supply the United States with intelligence about Iraq before the war, and over his efforts to position himself politically in Iraq after the invasion. Chalabi is accused of providing Iraqi defectors to the United States who in turn provided false assertions that Saddam Hussein possessed weapons of mass destruction. He is also criticized for convincing civilian Pentagon leaders that Iraqis would peacefully welcome U.S. forces as liberators. Until recently, the Defense Intelligence Agency paid Chalabi's group $340,000 a month for information.

Yesterday's Capitol Hill briefings came as Bush administration officials confirmed earlier reports that they are investigating allegations that Chalabi shared intelligence about the United States with Iran.

An investigation by the FBI was launched several weeks ago, officials said, after the United States intercepted a secret message from an Iranian intelligence agent in Baghdad who told his superiors in Tehran that Chalabi had revealed that Americans had cracked Iran's encryption code. The communication said a drunken American official gave Chalabi the information. At least five news organizations were tipped off to the inquiry over the past week but had held off publishing or broadcasting stories at the request of national security officials. The Washington Post was not among them.

The FBI is working to find out who first revealed that the United States had broken the code, and whether Chalabi was involved in passing on the information to Iran, officials said. Most of the officials interviewed for this article would speak only on the condition of anonymity because the investigation involves classified information.

One intelligence official said the FBI was conducting polygraph examinations. But senior officials at the Pentagon said they knew of no one there who had been interviewed by the FBI or who had been requested to submit to an interview. They also said the FBI had not informed top Pentagon authorities of plans to question any Defense Department employee in Washington.

A senior government official said yesterday that the investigation is "focusing on a person in Baghdad." Another official said "my sense is the FBI has a pretty good idea" who gave Chalabi the information.

As for how many might have known that Iran's code may have been broken, the senior government official said the number could be large.

"Every day, hundreds and even thousands of people read intelligence reports," the official said. "These people can deduce from reading intelligence reports that we're reading the mail of another country."

Another question yesterday was who leaked word of the intercepted Iranian message referring to Chalabi to reporters in Washington. "Only a handful of people knew about this specific intercept, and most of them were in the White House," one U.S. official said. "A few senior people at the Pentagon had been briefed on it. But very few people in the U.S. government had actually seen the piece of paper describing the intercept."

U.S. intelligence officials said the disclosure of the code-breaking would mean Tehran's security agencies will redo their codes and that for some time, perhaps years, American intelligence will not be able to read the transmissions.

The Bush administration considers Iran a potential threat to stability in the Middle East and is particularly worried about its efforts to develop nuclear weapons. Iran has been accused by nonproliferation officials at the United Nations of misstating details of its nuclear program and pursuing enrichment technology more aggressively than it admits.

The National Security Agency will now have to painstakingly try to break the new codes. Another technique is for the CIA to attempt to steal codebooks.

U.S. officials who have supported Chalabi defended him yesterday. And some officials raised the possibility that the incident could be an attempt by Iran to discredit Chalabi and remove him from contention as a future leader of Iraq.

"As a secular Shia and a democrat, he's a threat to Iran, which wants to see an Islamic government in Iraq," one official said. "Maybe these two Iranians were trying to set Chalabi up, knowing that the Americans would react viscerally if they suspected he had compromised codes."

The official also confirmed a report in this week's Time magazine about a National Security Council paper drafted in April on "marginalizing Chalabi." The official said that officials at the White House, the State Department and the CIA were upset with Chalabi because he opposed U.N. envoy Lakhdar Brahimi's mission to help form an interim Iraqi government and some U.S. policies in Baghdad.

The paper was brought up in a meeting of top administration officials, "but everybody got calmer heads and said, 'This is dumb. We should not do this. We don't want to make an enemy of him,' " the official said.

Rice discussed the allegations in a series of closed meetings yesterday on Capitol Hill.

In one session, Rice told the legislators that the FBI is investigating the matter, with a particular focus on who may have provided the code-breaking information to Chalabi. "Everyone in the room took it very seriously," said Sen. Mark Dayton (D-Minn.).

After hearing Rice, Sen. Chuck Hagel (R-Neb.) said: "We need to get the facts. These are pretty serious charges."

In an appearance on ABC's "Good Morning America," Rice said, "I can't comment on intelligence matters, but I can say that we have had a relationship with Mr. Chalabi in the past that's been strained recently."

In Najaf, Chalabi told the Associated Press allegations that he leaked highly classified information were "false" and "stupid." "Where would I get this from?" he asked. "I have no such information. How would I know anything about that?"

A senior administration official yesterday said that after the message about Chalabi was sent to Iran, the Iranians then transmitted an encrypted message in the same channel describing the location of an arms cache in Iraq, perhaps expecting the United States to follow up on it and thus confirm the code was broken. No action was taken by U.S. officials, who were hoping that their code-breaking would remain secret.

Richard N. Perle, the former chairman of the Defense Policy Board and a longtime supporter of Chalabi's, said yesterday he found it "impossible to believe" that Chalabi is accused of informing the Iranians about U.S. code-breaking and the station chief "would use a compromised code to report to Baghdad when he could convey it in 21/2 hours by car." Perle added: "It would be a tragedy if we jettisoned an Iraqi leader on such a hairy story."

Staff writers Helen Dewar, Bradley Graham and Thomas E. Ricks in Washington and correspondent Rajiv Chandrasekaran in Baghdad contributed to this report.