LOS ANGELES (AP) - Officials are investigating an explosion that killed a woman and injured three other people at a Southern California medical office building.

Orange County Sheriff's Department spokeswoman Carrie Braun says investigators worked through the night and were still at the scene Wednesday morning. A U.S. official told The Associated Press that the explosion is believed to have been intentional and may have come from a package.

The official was briefed on the investigation, was not authorized to discuss the matter publicly and spoke to The Associated Press on condition of anonymity. A sheriff's official said late Tuesday that no specific device had been found.