One person was killed and at least three others were injured in an explosion at a medical building in Aliso Viejo, authorities said.
A United States official told the Associated Press the authorities were investigating it as an intentional act and a package may have caused the blast.
Authorities initially indicated a vehicle had crashed into the building, but Orange County Fire Authority Capt.
Police investigators and FBI agents are studying the causes of the explosion, and they assure that there is no lingering threat to the community.
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.
The explosion, in a normally tranquil South Orange County city, prompted the evacuation of nearby businesses and a school filled with children, officials said.
"We are still trying to determine if this explosion was accidental or intentional", said Carrie Braun, a spokeswoman for the Orange County Sheriff's Office. The cause of the blast is being investigated.
Nothing indicates there were any threats made before the blast, which appeared to be concentrated in a suite on the first floor of the office building, Sawyer said.
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McWilliams says she saw two women covered in ash and soot stagger out of the building.
A third person suffered smoke inhalation, fire officials said. Initially it was reported that the auto crashed into the building, but later it became known that the explosion occurred inside the centre.
Shin said he and others hurried down the stairs to get out of that building, and saw "two holes in the wall" of the medical facility.
One woman, who had skin peeling on her arm from burns, said, "Take care of my mother", McWilliams said.
He explained that authorities were being extra cautious because they "don't see explosions in commercial buildings" often. The other woman was burned and her head was bleeding.
Two survivors were said to have critical injuries.
Orange County Fire Authority Chief Brian Fennessy says 10 engines and 70 firefighters were dispatched in response to a 1:09 p.m. Tuesday call from Aliso Viejo. The cause of the blast was being investigated.