One woman was killed and three others were injured during the Tuesday afternoon explosion at a medical office building in Aliso Viejo.
According to police, the explosion was a "deliberate detonation" - an explosive device was taken into the building and addressed to a specific person.
Police investigators and FBI agents are studying the causes of the explosion, and they assure that there is no lingering threat to the community.
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.
A daycare and preschool are near the site of the explosion, but authorities say all of the children are safe.
Authorities said they are still trying to figure out what happened and wouldn't comment on whether the incident could be related to a gas leak or explosive devices.
Mary McWilliams told the Orange County Register that she arrived for an appointment to find smoke surrounding the area and vehicle alarms going off.
Officials reportedly said the person who died is a woman and was near to the source of the explosion, The Orange County Register reported.
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McWilliams said she saw two burned women staggering out of the building, covered in ash and soot.
"There's heavy damage on the first floor corner of the building, walls and windows blown out", Capt.
A third person suffered smoke inhalation, fire officials said.
"Take care of my mother, take care of my mother!" shouted one woman as she fled, according to NBC4. They expected a lengthy investigation to determine the cause.
Authorities said they are not actively seeking a suspect. Other people could be scene wheeling cribs down the street as the area was evacuated. The other woman was bleeding from her head.
Two survivors were said to have critical injuries.
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.