The privacy policy typically bans the sharing of such confidential information, which includes home addresses, medical information and other personal data. But senior officials at the Social Security agency agreed to an "ad hoc" policy that authorized the release of information to the bureau for investigations related to Sept. 11 because officials saw a "life-threatening" emergency, internal memorandums say.

The Internal Revenue Service also worked with the bureau and the Social Security agency to provide income and taxpayer information in terror inquiries, law enforcement officials said. Officials said the I.R.S. information was limited because legal restrictions prevented the sharing of taxpayer information except by court order or in cases of "imminent danger" or other exemptions. The tax agency refused to comment.

The Social Security memorandums were obtained through a Freedom of Information Act request by the Electronic Privacy Information Center, a civil liberties group here. Copies were provided to The New York Times.

Social Security and law enforcement officials said that they were sensitive to privacy concerns and had put safeguards in place, but that they believed that the information gave investigators a valuable tool.

"We ran thousands of Social Security numbers," said a former senior F.B.I. official who insisted on anonymity because the files involved internal cases.

"We got very useful information, that's for sure," the former official said. "We recognized the value of having that information to track leads, and, to their credit, so did the Social Security Administration."

Some privacy advocates and members of Congress, although sympathetic to the extraordinary demands posed by the Sept. 11 investigation, said they were troubled by what they saw as a significant shift in privacy policies.

Representative Carolyn B. Maloney, a New York Democrat who has sought information from the Social Security agency on the issue, said the new policy had "real civil liberties implications for abuse." Ms. Maloney questioned whether Congress was adequately informed.

"If we don't know when the Social Security Administration decides to change its rules to disclose personal information," she said, "I think Americans have a right to be skeptical about their privacy."

The director of the Open Government Project at the Electronic Privacy Information Center, Marcia Hofmann, acknowledged the need for investigators to have access to vital information.

"But an ad hoc policy like this is so broad that it allows law enforcement to obtain really sensitive information by merely claiming that the information is relevant to the 9/11 investigation," Ms. Hofmann said. "There appears to be very little oversight."

In addition to easing its rules, the Social Security agency agreed to waive normal privacy restrictions for information related to the F.B.I. investigation of the sniper shootings in the Washington region in 2002, the internal memorandums show. It does not appear that any information was ultimately turned over.

The agency agreed two days after the Sept. 11 attacks to give the F.B.I. access to material from its files to obtain information on the hijackers, anyone with "relevant information" on the attacks and victims' relatives.

Under Social Security Administration policy, which goes beyond federal privacy law, such information cannot typically be shared with law enforcement officials unless the subject has been indicted or convicted of a crime. The loosening of the policy was updated and reauthorized last year, the internal documents show, and Social Security officials said Tuesday that it remained in place.

Social Security officials said they were not aware of recent F.B.I. requests for information from their files in the inquiry.

It appears that there was a flood of requests for at least a year after the attacks and perhaps longer, as bureau agents sought personal and financial information on illegal immigrants and other suspects who might be using fraudulent Social Security information. Some of the hijackers used fake Social Security numbers.

Officials at the Social Security Administration said the policy directives since Sept. 11 identified 11 offices - 10 in the inspector general's office and one in the privacy office - authorized to approve the F.B.I. requests.

"Thankfully, these requests don't come up that often," Jonathan Cantor, the privacy officer at the agency, said. "You just have to look at each situation as it comes in, and it's my job to balance the privacy of the records against legitimate requests for that information."

Mr. Cantor said that the bureau had made requests for financial and employment information from Social Security files, but that the agency had referred them to the I.R.S. because it was not legally allowed to release taxpayer information.

James Huse, who was the inspector general at the Social Security agency until March 2004, said his agency provided relevant identifying information to the bureau on possible terror suspects but relied on the tax agency to determine what information it would turn over on a suspect's income and employment.

The F.B.I. requests "came in by the thousands," Mr. Huse said in an interview. "They would give us the names of people suspected of being terrorists for whatever reason, and we'd match them against Social Security indices to see if these people were real, did they have Social Security numbers, things like that."

He said that most of the names and numbers run by the bureau did not match up to Social Security records, and that he was unaware of cases of governmental abuse in the requests. Mr. Huse added that "on a big-volume name check like that you wouldn't really know if you had a frivolous name thrown in or not."

A major debate is under way on access for the F.B.I. and other law enforcement agencies in national security investigations to confidential information like library checkout lists, hospital records or airline manifests. Bush administration officials say it is imperative for investigators to have broad tools to track terror suspects.