Here is what we know about the Tuesday blast and the investigation:
The powerful explosion took place just after 1 p.m. at a medical building undergoing renovation in Aliso Viejo, which is about 50 miles south of Los Angeles.
The blast, which blew out nearly the entire first floor, sent plumes of white smoke into the air and debris flying into the parking lot.
Witness Dong Shin told ABC News he heard a booming sound and then “felt the ground shake.”
When he saw fire and debris, Shin said, his first thought was a bomb.
“At first I was scared only because the people running around had a lot of fear in their faces,” he said.
Children at a day care facility across the street from the building were evacuated. Some were seen walking outside holding hands, while others were wheeled out in cribs.
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No children were hurt, officials said.
“It’s horrible, it’s a tragedy,” Orange County Fire Authority spokesman Tony Bommarito told ABC News. “Anytime you’re near this or you see it, it’s going to affect you.”
The scene has been rendered safe, the city said.
Searching for answers
The bomb squad combed the scene for answers Tuesday but officials have not yet said what led to the explosion.
The Orange County Sheriff’s Office said Tuesday night it had “not determined conclusively whether the explosion was accidental or intentional.”
There’s no “initial indication” of terrorism right now, Mike Gifford, an FBI spokesman in Los Angeles, said, though authorities will continue to investigate possible terrorism out of caution.