One person died and at least three were injured after the explosion at the Aliso Viejo medical building.
Officials said during a news conference that they hadn't "ruled out anything at all", in regards to how the explosion happened, according to ABC 7.
The explosion occurred on the first floor of a medical building at 11 Mareblu in Aliso Viejo in Orange County around 1:10 p.m. local time (2010 GMT), prompting a large-scale response by fire and sheriff's officials.
A fire that broke out in the building, which was under construction, was put out by crews, NBC reported.
By mid-afternoon, officials said they did not believe there was any further threat to the community.
An Orange County Sheriff's Department spokeswoman said: "We're trying to determine if a vehicle was involved or if it was an accident or inside the building".
"The investigation is ongoing", Braun told Xinhua by telephone.
He said investigators had not found an explosive device at the scene.
Bommarito said one of the injured people suffered from smoke inhalation, but the extent of the other victims' injuries was unknown.
Firefighters arrived and extinguished a fire, OCFA Capt.
Parents whose children were evacuated can reunite with them at the parking of the Target on La Paz Road, OCFA tweeted.
Fennessey says children at a daycare across the street were not injured.
"Then everyone started running out of buildings everywhere with looks on their faces of just horror", the woman said.