Investigators said they hadn't yet located an explosive device and were searching through debris.
The official, who was briefed on the investigation, was not authorized to discuss the matter publicly and spoke to The Associated Press on condition of anonymity.
The Bureau of Alcohol, Tobacco, Firearms and Explosives responded to the incident.
Associated Press writer Michael Balsamo in Los Angeles contributed to this report.
All nearby buildings including a preschool were evacuated by officials in the wake of the explosion.
Mary McWilliams informed The Register that she arrived for an appointment to seek out smoke surrounding the realm and automobile alarms going off. Two of the injured individuals had unknown accidents, and the third particular person suffered smoke-inhalation, in response to officers.
Crews discovered another leak after the explosion, a deputy told KTLA.
Orange County Sheriff's Cmdr.
Authorities initially indicated a vehicle had crashed into the building, but Bommarito said later that did not appear to be the case.
The address corresponds to a medical office building with multiple practices inside, online records show.
Mary McWilliams tells the Orange County Register that she arrived for an appointment Tuesday in Aliso Viejo to find smoke surrounding the area and vehicle alarms going off. The other woman was burned and her head was bleeding. Federal, state and local investigators were on the scene. Dave Sawyer said the size of the explosion made is suspicious.
He explained that authorities were being extra cautious because they "don't see explosions in commercial buildings" often.
The fire at the building at 11 Mareblu was quickly doused following the roughly 1:10 p.m. explosion.
The blast involved a building under renovation but the cause was not immediately known, Fennessy said.
Authorities did not actually enter the building until later in the afternoon, with a bomb squad sifting through the wreckage inside the structure.
The identity of the female victim who was killed, has yet to be released. The injured were taken to a hospital.
Fennessy said children at a day care across the street were not injured. They were being reunited with parents at a nearby store parking lot.
"The survivors' injuries were consistent with an explosion, but not necessarily consistent with a bomb".
Chief Fennesy noted the building where the explosion occurred was being renovated. They expected a lengthy investigation to determine the cause.