According to a written advisory, the security alert for all Justice Department facilities was triggered by the anniversary on August 7 of the bombings at two U.S. embassies in Africa, and not because of any specific threat. The alert says that anniversary dates are key indicators of possible terrorist activity.
The alert warns DOJ employees to report any suspicious persons or packages, to display their ID cards and to keep their offices secure.
The State Department has warned that a group it believes is led by Osama bin Laden, the Saudi exile suspected of orchestrating the embassy bombings, may be preparing to strike again.
The nearly simultaneous blasts in Nairobi, Kenya, and Dar es Salaam, Tanzania, killed 224 people, including 12 Americans, and injured thousands.
As the Justice Department warning was issued, U.S. security officials found traces of plastic explosives Thursday in a U.S. vehicle outside the American Embassy in Madagascar, an island nation off the southeast coast of Africa.
The jeep-type vehicle was stopped during a routine check at a roadblock installed in June to screen cars approaching the embassy, said an embassy security official, speaking on condition of anonymity.
Acting U.S. Ambassador June Carter Perry said traces of explosives showed up on new "highly sensitive" detection equipment at the embassy.
"We do not yet know if there is an explosive attached to the vehicle or if we have a false reading," she said.
The street outside the embassy in downtown Antananarivo was sealed off for several hours, and the vehicle was driven to an undisclosed location under police escort.
Perry said the embassy remained open and a decision would be made later on whether to close it for safety reasons.
The Madagascar embassy and five others in Africa were temporarily closed in June for security checks.