The official said the information was based on a preliminary investigation. Investigators said they hadn't yet located an explosive device and were searching through debris.
The powerful blast blew out almost the entire bottom floor, officials said.
Associated Press writer Michael Balsamo in Los Angeles contributed to this report. Tony Bommarito told CBS2.
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.
Orange County Sheriff's Cmdr. Dave Sawyer said Tuesday night that the size of the explosion makes it suspicious.
Firefighters put out a small fire and found one woman dead inside the building, the authorities said.
Mary McWilliams tells the Orange County Register that she arrived for an appointment Tuesday in Aliso Viejo to find smoke surrounding the area and auto alarms going off.
Shin said he and others hurried down the stairs to get out of that building, and saw "two holes in the wall" of the medical facility.
One woman, who had skin peeling on her arm from burns, said, "Take care of my mother", McWilliams said. The other woman was burned and her head was bleeding. The extent of their injuries is not yet known.
The identity of the female victim who was killed, has yet to be released. OCFA said a bomb squad was sent in to assess the situation and investigate the cause of the blast.
The blast occurred at an office building at around 1.25pm local time (9.25pm BST). All were taken to a hospital, she added. She and the three injured were likely in close proximity to the explosion, Sawyer said. Babies were seen being wheeled out of the facility in cribs, and toddlers held hands with each other and firefighters as they walked to a nearby Target for safety.
"The corner of that building, the whole bottom floor is pretty much blown out", fire Capt.
Fennessy says the blast involved a building under renovation.
Although the cause of the blast was still unknown, Bommarito told reporters around 3:30 p.m. that there was "no immediate hazard" to the community.