CBS News Correspondent Jim Stewart reports that the threat reportedly came from followers of Osama bin Laden, the wealthy Islamic radical who is charged in this country with masterminding last August's bombings of U.S. Embassies in Kenya and Tanzania. Those attacks killed 224 people, including 12 Americans.
Sources describe the latest bin Laden threat as "serious" and "credible" and say it is aimed directly at the heart of FBI operations -- the bureau's mammoth headquarters building on Pennsylvania Avenue in downtown Washington.
Fearful that tourists who gather there each day could be targeted, the bureau has ordered guided tours of the building postponed indefinitely.
The threat comes two weeks before the first anniversary of the attacks on two U.S. embassies in east Africa, and one month after bin Laden was named to the FBI's Ten Most Wanted list.
"The FBI continues to receive a high number of threats from individuals and organizations with ties to bin Laden," said FBI Director Louie Freeh.
Sources say the threats are not specific as to how such an attack might occur. Extra dog patrols and roving security guards will be added around the headquarters.
This latest threat has security analysts rattled because until now, bin Laden was believed to be concentrating on attacks against U.S. embassies and military bases overseas. To seriously plan a strike against FBI headquarters may mean he has sources in this country - sources that analysts hadn't previously known of.
FBI spokesman Tron Brekke said the tours, now taken annually by 250,000 people, would be suspended "indefinitely, until we can do a security assessment to ensure the safety of our employees and tourists."
"There is a threat against FBI facilities in Washington, D.C.," he said, adding that the threat is nonspecific and uncorroborated" -- that is, no specific target was identified. "Nevertheless, we have to take it seriously," he said.
The threat was not conveyed directly to the government by letter, telephone call or other communication, Brekke said. Rather it was discovered by intelligence means.
The FBI began opening its headquarters to public tours in 1937 when the bureau occupied part of the Justice Department building on Pennsylvania Avenue a few blocks from the White House. In 1975, the tours moved across the street to the new FBI headquarters in the J. Edgar Hoover Building. The tours are highlighted by agents demonstrating the firing of a submachine gun.