WASHINGTON — The FBI is giving significant new powers to its roughly 14,000 agents — allowing them more leeway to search databases, go through household trash or use surveillance teams to scrutinize the lives of people who have attracted their attention.

The FBI soon plans to issue a new edition of its manual, called the Domestic Investigations and Operations Guide, according to an official who has worked on the draft document and several others who have been briefed on its contents. The new rules add to several measures taken over the past decade to give agents more latitude as they search for signs of criminal or terrorist activity.

The FBI recently briefed several privacy advocates about the coming changes. Among them, Michael German, a former FBI agent who is now an attorney for the American Civil Liberties Union, argued it was unwise to further ease restrictions on agents’ power to use potentially intrusive techniques, especially if they lacked a firm reason to suspect someone of wrongdoing.

“Claiming additional authorities to investigate people only further raises the potential for abuse,” German said, pointing to complaints about the bureau’s surveillance of domestic political advocacy groups and mosques and to an inspector general’s findings in 2007 that the FBI had improperly used “national security letters” to obtain information like people’s phone bills.

Valerie Caproni, the FBI general counsel, said the bureau had fixed the problems with the national security letters and had taken steps to make sure they would not recur. She also said the bureau — which does not need permission to alter its manual so long as the rules fit within broad guidelines issued by the attorney general — had carefully weighed the risks and the benefits of each change.

Some of the most notable changes apply to the lowest category of investigations, called an “assessment.” The category, created in December 2008, allows agents to look into people and organizations “proactively” and without firm evidence for suspecting criminal or terrorist activity.