The blast at the two-storey building in Aliso Viejo, south of Los Angeles, also injured two people.
The blast reportedly blew siding of the walls of a medical office building, exposing insulation and framing as well as damaging a first floor suite.
A US official says a deadly explosion at a Southern California office building is being investigated as an intentional act and investigators believe the blast may have come from a package. A female of undisclosed age was killed, and three people were injured in the explosion at a medical building, authorities said.
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 powerful blast blew out almost the entire bottom floor, officials said.
However, sheriff's Cmdr. Dave Sawyer said the size of the explosion made it suspicious but no explosive device had been found, and it was unclear whether the blast was accidental or intentional.
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The Orange County Fire Authority said one person who was inside the building in Aliso Viejo died at the scene.
One person was pronounced dead at the scene, and three others were injured, OCFA said.
Children and staff at nearby Academy on the Hills preschool initially sheltered in place while the investigation continued, but children were later evacuated to a Target store parking lot so they could be reunited with their parents. Fire officials say a third person suffered smoke inhalation. McWilliams says one woman had skin peeling on her arm.
"We have confirmed there was no vehicle inside", Bommarito said, adding there was an explosion inside the building. The other woman was bleeding from her head.
Authorities said they are not actively seeking a suspect.
"The survivors' injuries were consistent with an explosion, but not necessarily consistent with a bomb".