A bomb exploded in an office building at 600 Pennsylvania Ave. SE yesterday morning, causing minor damage to the building but no injuries, according to police officials.

D.C. police said they have no information on who planted the explosive device, which was left near the entrance to the American Civil Liberties Union office on the third floor of the building. One woman was working in the ACLU office when the bomb went off at 9:55 a.m., police said, and there were a few people elsewhere in the building.

"It wasn't a gigantic explosion," said police department spokesman Sgt. Joe Gentile. He said a door was knocked down and plaster fell from the wall in the corridor, but the explosion did not cause a fire.

Police did not know if the bomb was aimed at the ACLU office or some other occupant of the building. The congressional Office of Technology Assessment occupies several offices in the five-story building, which also holds the National Permanent Bank and Riggs National Bank. In addition to the ACLU, there are three medical offices on the third floor, according to an occupant of the building.

"This dude came by me right fast and then there was a boom," said Northeast resident John Clark, who was sitting on the first-floor stairs waiting for a doctor's appointment when he heard the blast. "It sounded like somebody laid a hand grenade down."