WASHINGTON  A Justice Department plan would loosen restrictions on the Federal Bureau of Investigation to allow agents to open a national security or criminal investigation against someone without any clear basis for suspicion, Democratic lawmakers briefed on the details said Wednesday.

The plan, which could be made public next month, has already generated intense interest and speculation. Little is known about its precise language, but civil liberties advocates say they fear it could give the government even broader license to open terrorism investigations.

Congressional staff members got a glimpse of some of the details in closed briefings this month, and four Democratic senators told Attorney General Michael B. Mukasey in a letter on Wednesday that they were troubled by what they heard.

The senators said the new guidelines would allow the F.B.I. to open an investigation of an American, conduct surveillance, pry into private records and take other investigative steps “without any basis for suspicion.” The plan “might permit an innocent American to be subjected to such intrusive surveillance based in part on race, ethnicity, national origin, religion, or on protected First Amendment activities,” the letter said. It was signed by Russ Feingold of Wisconsin, Richard J. Durbin of Illinois, Edward M. Kennedy of Massachusetts and Sheldon Whitehouse of Rhode Island.