SAN DIEGO — An early morning explosion at the federal courthouse in downtown San Diego left a door damaged and blew out a window. Sgt. Bob Dare of the San Diego Police Department says the loud blast occurred at about 1:40 a.m. Sunday. Streets in the area have been closed. No injuries were reported. The explosion is under investigation. FBI Special Agent Darrell Foxworth told a local TV station it appears the explosion may have been caused by a pipe bomb.