The documents indicate that the FBI used secret demands for records to obtain data not only on the person it was targeting but also details on his or her "community of interest" - the network of people that the target in turn was in contact with. The FBI recently stopped the practice in part because of broader questions raised about its aggressive use of the demands made through national security letters, officials said Friday after being asked about it.

The community-of-interest data sought by the FBI is central to a data-mining technique intelligence officials call "link analysis." Since the attacks of Sept. 11, 2001, U.S. counterterrorism officials have turned more frequently to the technique, using communications patterns and other data to identify suspects who may not have any other known links to extremists.

The concept has strong government proponents who see it as a vital tool in predicting and preventing attacks, and it is also thought to have helped the National Security Agency identify targets for its domestic eavesdropping program. But privacy advocates, civil rights leaders and even some counterterrorism officials warn that link analysis can be misused to establish tenuous links to people who have no real connection to terrorism but may be drawn into an investigation nonetheless.

Typically, community-of-interest data might include an analysis of which people the targets called most frequently, how long they generally talked and at what times of day, sudden fluctuations in activity, geographic regions that were called and other data, law enforcement and industry officials said.

The FBI declined to say exactly what data had been turned over. The data was limited to people, phone numbers and e-mail "once removed" from the actual target of the national security letters, said a government official spoke on condition of anonymity because of a continuing review by the Justice Department.

The scope of the demands for information could be seen, for instance, in an August 2005 letter seeking the call records for particular phone numbers that had come under suspicion. The letter closed by saying: "Additionally, please provide a community of interest for the telephone numbers in the attached list."

The requests for community-of-interest data showed up a dozen times, using nearly identical language, in records from one six-month period in 2005 obtained by a nonprofit advocacy group, the Electronic Frontier Foundation, through a Freedom of Information Act lawsuit that it brought against the government. The FBI recently turned over 2,500 pages of documents to the group.

The boilerplate language suggests the requests may have been used in many of more than 700 emergency or "exigent" national security letters. Earlier this year, the FBI banned the use of the exigent letters because they had never been authorized by law.

The FBI declined to discuss any aspect of the community-of-interest requests because it said the issue was part of an investigation by the Justice Department inspector general's office into national security letters. An initial review in March by the inspector general found widespread violations and possible illegality in the FBI's use of the letters, but it did not mention the use of community-of-interest data.

The government official who spoke on condition of anonymity said the FBI recently stopped asking the telecommunications companies for community-of-interest data. The exact time of and reason for the suspension is unclear, but it appears to have been triggered in part by the questions raised this year by the inspector general's initial review into abuses in the use of national security letters.

The official said the FBI itself was examining the use of the community-of-interest requests to get a better understanding of how and when they were used, but he added that they appeared to have been used in a relatively small portion of the tens of thousands of such records requests each year. "In an exigent circumstance, that's information that may be relevant to an investigation," the official said.

A federal judge in New York struck down parts of the USA Patriot Act last week that authorized the FBI's use of the national security letters, saying that some provisions violated the First Amendment and the constitutional guarantee of the separation of powers.

In many cases, the target of a national security letter whose records are being sought is not necessarily the actual subject of a terrorism investigation and may not be suspected at all. Under the Patriot Act, the FBI must assert only that the records gathered through the letter are considered relevant to a terrorism investigation.

Some legal analysts and privacy advocates suggested that the disclosure of the FBI's collection of community-of-interest records - extending the link even further beyond an actual suspect in a terrorism investigation - offered another example of the bureau's exceeding the substantial powers already granted it by Congress.

"This whole concept of tracking someone's community of interest is not part of any established FBI authority," said Marcia Hofmann, a lawyer for the Electronic Frontier Foundation, which provided the records from its lawsuit. "It's being defined by the FBI. And when it's left up to the FBI to decide what information is relevant to their investigations, they can vacuum up almost anything they want"

Matt Blaze, a professor of computer and information science at the University of Pennsylvania and a former researcher for AT&T, said the telecommunications companies could have easily provided the FBI with the type of network analysis data it was seeking because they themselves had developed it over many years.

"This sort of analysis of calling patterns and who the communities of interests are is the sort of things telephone companies are doing anyway because it's central to their businesses for marketing or optimizing the network or detecting fraud," said Blaze, who has worked with the FBI on technology issues.

Such "analysis is extremely powerful and very revealing because you get these linkages between people that wouldn't be otherwise clear, sometimes even more important than the content itself" of phone calls and e-mail messages, he said. "But it's also very invasive. There's always going to be a certain amount of noise," with data collected on people who have no real links to suspicious activity, he said.

Officials at other U.S. intelligence agencies, like the National Security Agency and the Central Intelligence Agency, have explored using link analysis to trace patterns of communications sometimes two, three or four people removed from the original targets, according to current and former intelligence officials. But critics assert that the further the links are taken, the less valuable the information proves to be.

Some privacy advocates said they were troubled by what they saw as the FBI's overreliance on technology at the expense of traditional investigative techniques that rely on clearer evidence of wrongdoing.

"Getting a computer to spit out a hundred names doesn't have any meaning if you don't know what you're looking for," said Michael German, a former FBI agent who is now a lawyer for the American Civil Liberties Union . "If they're telling the telephone company, 'You do the investigation and tell us what you find,' the relevance to the investigation is being determined by someone outside the FBI."

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