Carrie Braun, an Orange County Sheriff's Division spokeswoman, stated authorities had been working to find out if the explosion was an accident or an intentional act, the newspaper reported.
Early reports indicated that a vehicle may have crashed into the building, causing the explosion, but authorities soon ruled that out.
No children were hurt, officials said.
Bommarito said there didn't appear to be a gas leak. Two other people, believed to be patrons, suffered injuries which required surgery and are expected to survive.
Paul Delacourt, the assistant director in charge of the FBI's field office in Los Angeles, described the explosion site as a "crime scene", but did not elaborate, according to the AP.
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. One of the officials said one of the victims was believed to be the target.
The officials said the explosion did not appear to be part of a broader plot and there was no immediate connection to any extremist groups. Police investigators and FBI agents are studying the causes of the explosion, and they assure that there is no lingering threat to the community.
Federal and local investigators searched through debris for remnants of a possible explosive device at a southern California medical office building that exploded Tuesday afternoon.
In the early morning hours Wednesday, FBI investigators began searching a home in Trabuco Canyon were Krajnyak was believed to have been living with her estranged husband.
A US official told The Associated Press that the explosion is believed to have been intentional and may have come from a package.
"We have not found any type of specific device inside of the building right now that would tell us or lead us to exactly what the device was, if there was a device", sheriff's Cmdr.
The victim, who was confirmed dead on the scene, has not yet been named.
The Tuesday afternoon blast occurred in Aliso Viejo, which is about 50 miles south of Los Angeles. They said they believe the explosion came from inside the building, Bommarito said, since the doors and windows on the bottom floor were "blown out". They had not released her name as of late Tuesday night. He would not say whether the woman killed had been specifically targeted.
Dong Shin said he had seen "fire, smoke, insulation popping out of the walls, a lot of scared people running around, a lot of commotion".
Shorly after the blast, Mary McWilliams of San Clemente arrived for an appointment to find smoke surrounding the area and auto alarms going off.
There's no word yet on a motive for the explosion.