Known terrorists have entered the United States, the FBI chief warned, and security in the nation's capital was tightened to record levels in the wake of war in the Middle East.

FBI Director William Sessions said terrorists had entered the country but there was "no way" to determine their "intent or activity."During an interview Wednesday night with CBS News, Sessions said, "Yes, there are elements of those terrorist groups that have been identified as being in the United States." However, he did not elaborate on their whereabouts.

An FBI spokesman said Thursday that when Sessions referred to "elements" of terrorist groups, he meant either terrorist "individuals or groups." The spokesman also said the FBI is continuing its controversial interviews with Arab-American leaders, criticized by civil rights groups as targeting people solely on the basis of ethnic background.

Stringent security precautions were in effect at the White House, on Capitol Hill and at the Pentagon, State Department, Justice Department and other government agencies.

Uniformed White House police posted inside the gates wore riot gear and also carried gas masks on their belts. An official said the gas masks were likely being worn as a precaution in the event tear gas was used against demonstrators across the street.

White House tours were canceled for the first time since the Vietnam War. A popular Washington attraction, the White House tours normally attract some 3,500 people a day.

The White House press office said the tours were halted during World War I and World War II, then stopped again "periodically" during the Vietnam War in the 1960s and 1970s.

"The tours offer the most immediate opportunity for unwarranted intrusion into the White House," said presidential spokesman Marlin Fitzwater. There have been almost daily anti-war protests near the White House.

At the Pentagon, public tours were restricted. Employees who normally enter after their credentials are checked also were required to pass through metal detectors.

At the State Department, all streets were blocked off at the moment the U.S. bombing campaign began against in Iraq and Kuwait. The streets were blocked by black security vans.

The security was relaxed slightly Thursday, but "tank-trap" gates were operating at all garage entrances, and security patrols were stepped up dramatically.

At the Justice Department, many entrances were sealed and employee identifications were checked carefully and compared with parking permits.

Other government agencies stepped up security, including the Agriculture Department, where it had been relatively lax.

There also were many bomb threats around the city, but police searches turned up no explosive devices. Several offices of a Republican senator from New York also received telephone bomb threats.

Security also was tightened at airports around the country - causing travel delays - and curbside baggage checks were eliminated. More than a million people use the U.S. air transportation system every day, making it a visible target.