Washington ? National security adviser Condoleezza Rice on Wednesday 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 Wednesday. “A very valuable tool the United States had was taken away.”

Iraqi politician Ahmad Chalabi, whose exile group received more than $40 million in U.S. payments over the years, denied Wednesday 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 Atty. Gen. John Ashcroft and FBI Director Robert Mueller, 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 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.

Wednesday’s Capitol Hill briefings came as administration officials confirmed earlier reports that they were investigating charges that Chalabi shared U.S. intelligence 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.