Instead of collecting information only on people about whom they had a tip or links to the teenager, agents fanned out to scrutinize Somali communities, including in Seattle, Washington, and Columbus, Ohio. The operation unfolded as the Bush administration was relaxing some domestic intelligence-gathering rules.

The F.B.I.’s interpretation of those rules was recently made public when it released, in response to a Freedom of Information lawsuit, its “Domestic Investigations and Operations Guide.” The disclosure of the manual has opened the widest window yet onto how agents have been given greater power in the post-Sept. 11 era.

In seeking the revised rules, the bureau said it needed greater flexibility to hunt for would-be terrorists inside the United States. But the manual’s details have alarmed privacy advocates.

One section lays out a low threshold to start investigating a person or group as a potential security threat. Another allows agents to use ethnicity or religion as a factor - as long as it is not the only one - when selecting subjects for scrutiny.