The Federal Bureau of Alcohol, Tobacco and Firearms joined with the F.B.I. and the police in the investigation. A law enforcement official said a Postal Service worker and a window washer got a good look at a man as he kneeled and placed something on the ground between two pillars at the side entrance.

The official said that four other post office employees, who like their colleague were on their way to work at the Peck Slip Station, passed by the device, and that one of them saw what he thought was a burning fuse.

''One of them sees this thing on the ground with what looks like a lit fuse, says 'Bomb!' or something, and they run for it,'' the official said.

They were about 30 feet from the bomb, just rounding a corner, when it exploded, said Mr. Kelleher, who joined the mayor at the midday briefing because Police Commissioner Howard Safir was away on vacation. One of the postal workers was treated for ringing in his ears and released.

Investigators were checking videotape from security cameras in the area -- one of the most closely monitored areas in the city -- to see if they picked up the bombing or the movements of a possible suspect before or after the explosion. Detectives also ordered MetroCard records from nearby subway stations so that they could make possible links to any suspect who might have traveled by train.

Mr. Giuliani said authorities were offering an $11,000 reward for information leading to the conviction of the bomber. The police described the suspect as a 35- to 40-year-old light-skinned black or Hispanic man, about 5-foot-10 inches to 6 feet tall and weighing 200 to 225 pounds. Some witnesses said the man, who was wearing a black cap, was carrying a yellow toolbox.

Investigators said the green metal ammunition can was packed with black powder or smokeless powder and cardboard.