A deputy with Orange County Sheriff's Department told KTLA the explosion was caused by a gas leak and the area is being evacuated. There were early reports that a auto had crashed into the building, but that turned out not to be the case.
Officials haven't released information about the person who was killed.
The explosion occurred on the first floor of a medical building at 11 Mareblu in Aliso Viejo in Orange County around 1:10 p.m. local time (2010 GMT), prompting a large-scale response by fire and sheriff's officials. No children were hurt, officials said. Investigators are interviewing the three injured.
Authorities say one person is dead and two others have injuries after an explosion rocked a Southern California office building.
"We are going to take every precaution necessary before we enter that building", Bommarito said. OCFA said a bomb squad was sent in to assess the situation and investigate the cause of the blast.
"Anytime you see an explosion of this magnitude, it would definitely be suspicious to us and that's why we rolled out all the resources to get to the bottom of it", he said.
The powerful blast blew out almost the entire bottom floor, officials said.
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The bomb was reportedly addressed to a specific person.
The FBI is responding to the scene, a law enforcement official told CBS News senior investigative producer Pat Milton.
There was no initial indication of terrorism, according to the Federal Bureau of Investigation.
Dong Shin, a witness, said he heard a loud boom minutes before his appointment in a nearby building. "My whole body shook".
He said it felt "almost like an natural disaster, but a big quake".
The explosion at the office building in Aliso Viejo - about 50 miles (80.5 kilometers) south of Los Angeles - blew siding off the walls, exposing insulation and framing. Her face was covered in blood, he said.