The FBI was sending resources to the scene, but agency spokesperson Mike Gifford said there was no early indication of terrorism, CNN reporter Kristen Holmes tweeted.
He said investigators had not found an explosive device at the scene.
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. Firefighters arrived and extinguished a fire, OCFA Capt.
"As far as what I'm hearing there was no vehicle involved", Carrie Braun of the Orange County Sheriff's Department said.
An explosion at a medical office building in Southern California killed a woman and injured three people.
Initial reports of the explosion claimed a auto crashed into the building around 1 p.m. and that the explosion was not considered suspicious.
The county's bomb squad and a hazmat team are preparing to search the building where the explosion occurred. Mr. Bommarito said all three were expected to survive.
Law enforcement ordered nearby Academy on the Hills day care and preschool to evacuate and children were rolled in cribs and walked hand-in-hand down the street. The nature of their injuries were not immediately known. The person who died was inside the building at the time of the blast, officials said.
"Then everyone started running out of buildings everywhere with looks on their faces of just horror", the woman said. ". I saw two women come out full of blood, hair singed, just glass stuck to the hair, glass stuck to their bodies". Their skin was burned and peeled back, and they were just in shock.
That building is located at 11 Mareblu, OCFA tweeted.