WASHINGTON — After a Somali-American teenager from Minneapolis committed a suicide bombing in Africa in October 2008, the Federal Bureau of Investigation began investigating whether a Somali Islamist group had recruited him on United States soil.

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 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.

“It raises fundamental questions about whether a domestic intelligence agency can protect civil liberties if they feel they have a right to collect broad personal information about people they don’t even suspect of wrongdoing,” said Mike German, a former F.B.I. agent who now works for the American Civil Liberties Union.

But Valerie Caproni, the F.B.I.’s general counsel, said the bureau has adequate safeguards to protect civil liberties as it looks for people who could pose a threat.

“Those who say the F.B.I. should not collect information on a person or group unless there is a specific reason to suspect that the target is up to no good seriously miss the mark,” Ms. Caproni said. “The F.B.I. has been told that we need to determine who poses a threat to the national security — not simply to investigate persons who have come onto our radar screen.”

The manual authorizes agents to open an “assessment” to “proactively” seek information about whether people or organizations are involved in national security threats.

Agents may begin such assessments against a target without a particular factual justification. The basis for such an inquiry “cannot be arbitrary or groundless speculation,” the manual says, but the standard is “difficult to define.”

Assessments permit agents to use potentially intrusive techniques, like sending confidential informants to infiltrate organizations and following and photographing targets in public.

F.B.I. agents previously had similar powers when looking for potential criminal activity. But until the recent changes, greater justification was required to use the powers in national security investigations because they receive less judicial oversight.

If agents turn up something specific to suggest wrongdoing, they can begin a “preliminary” or “full” investigation and use additional techniques, like wiretapping. But even if agents find nothing, the personal information they collect during assessments can be retained in F.B.I. databases, the manual says.

When selecting targets, agents are permitted to consider political speech or religion as one criterion. The manual tells agents not to engage in racial profiling, but it authorizes them to take into account “specific and relevant ethnic behavior” and to “identify locations of concentrated ethnic communities.”

Farhana Khera, president of Muslim Advocates, said the F.B.I. was harassing Muslim-Americans by singling them out for scrutiny. Her group was among those that sued the bureau to release the manual.

“We have seen even in recent months the revelation of the F.B.I. going into mosques — not where they have a specific reason to believe there is criminal activity, but as ‘agents provocateurs’ who are trying to incite young individuals to join a purported terror plot,” Ms. Khera said. “We think the F.B.I. should be focused on following actual leads rather than putting entire communities under the microscope.”

Ms. Caproni, the F.B.I. lawyer, denied that the bureau engages in racial profiling. She cited the search for signs of the Somali group, Al Shabaab, linked to the Minneapolis teenager to illustrate why the manual allows agents to consider ethnicity when deciding where to look. In that case, the bureau worried that other such teenagers might return from Somalia to carry out domestic operations.

Agents are trained to ignore ethnicity when looking for groups that have no ethnic tie, like environmental extremists, she said, but “if you are looking for Al Shabaab, you are looking for Somalis.”

Among the manual’s safeguards, agents must use the “least intrusive investigative method that effectively accomplishes the operational objective.” When infiltrating an organization, agents cannot sabotage its “legitimate social or political agenda,” nor lead it “into criminal activity that otherwise probably would not have occurred.”

Portions of the manual were redacted, including pages about “undisclosed participation” in an organization’s activities by agents or informants,”‘requesting information without revealing F.B.I. affiliation or the true purpose of a request,” and using “ethnic/racial demographics.”

The attorney general guidelines for F.B.I. operations date back to 1976, when a Congressional investigation by the so-called Church Committee uncovered decades of illegal domestic spying by the bureau on groups perceived to be subversive — including civil rights, women’s rights and antiwar groups — under the bureau’s longtime former director, J. Edgar Hoover, who died in 1972.

The Church Committee proposed that rules for the F.B.I.’s domestic security investigations be written into federal law. To forestall legislation, the attorney general in the Ford administration, Edward Levi, issued his own guidelines that established such limits internally.

Since then, administrations of both parties have repeatedly adjusted the guidelines.

In September 2008, Attorney General Michael B. Mukasey signed the new F.B.I. guidelines that expanded changes begun under his predecessor, John Ashcroft, after the Sept. 11 attacks. The guidelines went into effect and the F.B.I. completed the manual putting them into place last December.

There are no signs that the current attorney general, Eric H. Holder Jr., plans to roll back the changes. A spokeswoman said Mr. Holder was monitoring them “to see how well they work” and would make refinements if necessary.

The F.B.I., however, is revising the manual. Ms. Caproni said she was taking part in weekly high-level meetings to evaluate suggestions from agents and expected about 20 changes.

Many proposals have been requests for greater flexibility. For example, some agents said requirements that they record in F.B.I. computers every assessment, no matter how minor, were too time consuming. But Ms. Caproni said the rule aided oversight and would not be changed.

She also said that the F.B.I. takes seriously its duty to protect freedom while preventing terrorist attacks. “I don’t like to think of us as a spy agency because that makes me really nervous,” she said. “We don’t want to live in an environment where people in the United States think the government is spying on them. That’s an oppressive environment to live in and we don’t want to live that way.”

What the public should understand, she continued, is that the F.B.I. is seeking to become a more intelligence-driven agency that can figure out how best to deploy its agents to get ahead of potential threats.

“And to do that,” she said, “you need information.”