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. The powerful blast also damaged the lobby area of the Edward J. Schwartz Federal Courthouse about, said FBI spokeswoman April Langwell. The bomb was reported by two guards in the building. About 40 FBI agents combed the front courtyard after the area was swept for explosives. No arrests have been made. Debris from the explosion was found lodged in a window about eight stories up the AT&T building that faces the courthouse. Authorities said that surveillance cameras outside the building may provide information, but that no cameras directly face the doorway.