San Diego ? A suspected pipe bomb exploded at a federal courthouse in downtown San Diego early Sunday, damaging the front entrance and blowing out a window, authorities said. No injuries were reported.

Few people were around the building, which is a block from nightclubs in the Gaslamp Quarter, when the powerful blast also damaged the lobby area of the Edward J. Schwartz Federal Courthouse about 1:40 a.m., said FBI spokeswoman April Langwell.

The bomb was reported by two guards in the building, who were uninjured. About 40 agents combed the front courtyard after the area was swept for explosives. No arrests have been made.