The F.B.I. cast a much wider net in its terrorism investigations than it has previously acknowledged by relying on telecommunications companies to analyze phone-call patterns of the associates of Americans who had come under suspicion, according to newly obtained bureau records.
The documents indicate that the Federal Bureau of Investigation used secret demands for records to obtain data not only on individuals it saw as targets but also details on their “community of interest” — the network of people that the target was in contact with. The bureau stopped the practice early this year in part because of broader questions raised about its aggressive use of the records demands, which are known as national security letters, officials said.
The community of interest data sought by the F.B.I. is central to a data-mining technique intelligence officials call link analysis. Since the attacks of Sept. 11, 2001, American 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.