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 ‘agent 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.”