Bomb technicians from the Orange County Sheriff's Department, the Federal Bureau of Investigation and other agencies were investigating, said Paul Delacourt, the assistant director in charge of the FBI's field office in Los Angeles.
"We have not found any type of specific device inside of the building right now that would tell us or lead us to exactly what the device was - if there was a device", Sawyer said. It went off around 1 p.m.
Carrie Braun, an Orange County Sheriff's Department spokeswoman, said authorities were working to determine if the explosion was an accident or an intentional act, the newspaper reported. Investigators said they hadn't yet located an explosive device and were searching through debris.
Authorities initially believed a vehicle had crashed into the side of the structure but later said that was not the case.
The powerful blast blew out almost the entire bottom floor, officials said. OCFA said a bomb squad was sent in to assess the situation and investigate the cause of the blast. Mr. Bommarito said all three were expected to survive.
Sky5 footage showed crews evacuating people-including children, some in cribs-out of the Academy on the Hills, a preschool across the building where the explosion happened.More news: Trump visits first lady at Walter Reed hospital
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Officials reportedly said the person who died is a woman and was near to the source of the explosion, The Orange County Register reported.
"As far as what I'm hearing there was no auto involved", Carrie Braun of the Orange County Sheriff's Department said.
Fennessy says children at a daycare across the street were not injured.
A woman who was on her way to an appointment in the area told CBS2 she arrived just after the blast occurred, and she saw fire on the roof of the building.
They said they were not actively looking for any suspects. One of those three suffered smoke inhalation. "... I saw two women come out full of blood, hair singed, just glass stuck to the hair, glass stuck to their bodies". A woman was pronounced dead at the scene, and three others were injured, OCFA said. It was not immediately clear what prompted that call.