Dong Shin said he had seen "fire, smoke, insulation popping out of the walls, a lot of scared people running around, a lot of commotion". The blast involved a building under renovation but the cause was not immediately known, according to authorities.
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.
The explosion, in a normally tranquil South Orange County city, prompted the evacuation of nearby businesses and a school filled with children, officials said.
Mary McWilliams told The Register that she arrived for an appointment to find smoke surrounding the area and auto alarms going off.
The Tuesday afternoon blast on the first floor of an Aliso Viejo building in Orange County shattered windows and forced children in a nearby daycare center to be evacuated.
McWilliams says she saw two women covered in ash and soot stagger out of the building.
In a news conference Tuesday night, Orange County Sheriff's Department Commander Dave Sawyer said it was unclear whether the explosion was intentional or accidental.
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One woman, who had skin peeling on her arm from burns, said, "Take care of my mother", McWilliams said.
They said they were not actively looking for any suspects.
Although the cause of the blast was still unknown, Bommarito told reporters around 3:30 p.m. that there was "no immediate hazard" to the community. The other woman was bleeding from her head.
There was no immediate information about the person who was killed.
Braun said the two survivors had critical injuries "that were consistent with an explosion, but not necessarily consistent with a bomb".
Brian Fennessy, the Orange County Fire Authority chief, said 70 firefighters were dispatched in response to the call in Aliso Viejo at 1:09 p.m. local time. The medical center is visited by a huge number of families, but none of the children were not injured.