Orange County Sheriff's Cmdr.
The officials say there have been no arrests.
"At this time, we do not know how the device arrived at the location", Delacourt said.
The explosion blew siding off the walls, exposing insulation and framing.
A powerful explosion tore through an Aliso Viejo office building that houses a medical office on Tuesday afternoon, killing a woman, injuring three other people and drawing a swarm of law enforcement officers who blanketed the scene amid a throng of curious and concerned onlookers. An intense investigation including local law enforcement - but also involving members of the FBI and other federal agencies - was immediately set into motion.
"We have not confirmed that the explosion is intentional, nor have we conclusively identified the source of the explosion".
After the explosion, bomb squad investigators arrived on the scene to search for more risky devices, while children at a nearby day care were evacuated. The building sits in the shadow of the 73 toll road, which cuts through south Orange County, and across the street from a day-care center.
Dong Shin, a witness, said he heard a loud boom minutes before his appointment in a nearby building.
"And the ground shook", he told CNN.
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Orange County Undersheriff Don Barnes says the formal identification of the body was continuing but authorities believe it is that of Ildiko Krajnyak (IL-di-koh KRY-nyak), a licensed cosmetologist. The three people who were injured were nearby and were being questioned by investigators late Tuesday night, he said. Investigators are interviewing the three injured.
Officials say other potential causes such as a gas leak have been ruled out.
Shin said it felt like "a big quake", and as he and others rushed downstairs to get out of the nearby building, he saw two holes in the wall of the medical facility. Two of the injured individuals had unknown accidents, and the third particular person suffered smoke-inhalation, in response to officers.
One woman who ran out of the medical facility had her face covered in blood, Shin said.
At this point, officials are not ruling anything out.
Firefighters put out a small fire and found one woman dead inside the building, the authorities said. "Anytime you're near this or you see it, it's going to affect you".
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. There is no specific person that authorities are searching for right now, he added.
There's no "initial indication" of terrorism right now, Mike Gifford, an Federal Bureau of Investigation spokesman in Los Angeles, said, though authorities will continue to investigate possible terrorism out of caution.