A suspicious package was detonated by authorities early this morning at FBI headquarters in Old Town Alexandria after the surrounding area was cordoned off for more than two hours.

An FBI spokesman said that the FBI had been almost certain that the package, measuring about nine inches by six inches by six inches and found beneath an automobile in the ground-floor headquarters garage, did not contain explosives.

But after initial examination, which included X-ray inspection, "we thought it best to detonate it with a small charge" at the headquarters at 300 N. Lee St., agent Joseph Krahling said.

Krahling said the detonation, carried out an Army bomb unit from Fort Belvoir, indicated that the package contained no explosive.

The contents of the package could not be learned immediately. Krahling said they were being analyzed early this morning.

Authorities said the package was found about 10:15 p.m. after the FBI received an anonymous telephone call telling of it. Few details about the call were disclosed, but Krahling said that "the implication was that the package was dangerous."

Authorities said they believed that the package had been thrown into the garage through a grating.

The bomb squad was called, and after the examination of the package, which included removing part of the wrapping, the car under which it was found was moved. Sandbags were placed around the package, and it was detonated about 12:40 a.m. People in the vicinity said they heard what sounded like a muffled explosion.

Patrons of the nearby Bilbo Baggins restaurant were forced to remain inside well past closing time, some playing charades, until after the detonation, a waitress said.

Pat Fitzgibbon, who recently moved into the neighborhood, said an FBI agent went to her door about 11 and suggested that she and her daughter leave temporarily. She said she returned in about an hour and was told that "we should go drive around some more." She finally returned after the detonation.