The blast at the two-storey building in Aliso Viejo, south of Los Angeles, also injured two people.
The explosion came from the inside out, and the first floor is heavily damaged.
Investigators believe the Tuesday blast, which happened in Orange County, may have come from a package, according to the Associated Press.
Officials didn't consider the explosion suspicious and they found no explosive devices.
Authorities say explosives experts are trying to determine what caused an explosion that killed a woman and injured three other people at a Southern California medical office building.
FBI spokesman Mike Gifford said the cause of the explosion is still unknown and the agency has sent resources to help with the investigation, which is being led by Orange County Sheriff's Department, local news outlet City News Service reported.
The blast involved a building under renovation, Fennessy said.
Facebook page on Aug. 5 2017
The Orange County Fire Authority (OCFA) said the explosion occurred on the first floor.
The dead and injured have not been named.
Officials originally believed the explosion was caused by a vehicle crashing into the building, but Orange County Sheriff's Department spokeswoman Carrie Braun said no auto was involved and the department was investigating how the explosion occurred.
The FBI also said there was no indication of terrorism, but it would continue to investigate out of caution.
Mary McWilliams told the Orange County Register that she arrived for an appointment to find smoke surrounding the area and auto alarms going off. Fire officials say a third person suffered smoke inhalation.
They said they were not actively looking for any suspects. Still, they chose to evacuate a nearby day care and other surrounding businesses as a precaution. The other woman was burned and her head was bleeding.
Two survivors had critical injuries "that were consistent with an explosion, but not necessarily consistent with a bomb", said Carrie Braun, a spokeswoman for the Orange County Sheriff's Department.More news: Salmonella outbreak: More illnesses linked to recalled eggs reported in Virginia