A suspected pipe bomb exploded at the federal courthouse in downtown San Diego early Sunday, damaging a door and blowing out a window, authorities said.

The loud blast occurred at about 1:40 a.m. at the Edward J. Schwartz Federal Courthouse, said FBI spokeswoman April Langwell. The device caused damage to the front entrance and lobby area of the courthouse, she said.

The bomb was reported by two guards inside the building, who were uninjured. About 40 agents combed the courtyard in front of the courthouse after the area was swept for explosives.

Debris was found as far away that was lodged in a window about eight stories up the AT&T building that faces the courthouse.

Authorities said surveillance cameras outside the building may provide information, but there are no cameras that directly face the doorway. At the time of the explosion there were not many people around the building, which is a block from nightclubs in the Gaslamp Quarter.

Streets in the area have been closed.

No injuries were reported. The explosion is under investigation and no arrests have been made.