The fire erupted inside the Maxim Healthcare building at 17291 Irvine Boulevard at about 7:30 a.m. Maxim Healthcare specializes in senior health care.
"All of a sudden, the glass just burst out of the first floor window, and flames immediately started coming out," said witness Ryan Clear, a Maxim Healthcare employee.
Firefighters had difficulties dealing with the rare 6-alarm fire, six times their regular response.
"There were people inside the building at the time the fire was reported. Luckily, pretty quickly after we got on scene, we were able to call what we can an 'all clear' and effectively get everybody out," said Orange County Fire Battalion Chief Kris Concepcion.
No patients were inside, but the building was filled with years of medical files.
"All of our records were in there, either on the computer or on paper, so they're up in flames," Clear said.
The flames were so intense that at one point, firefighters had to back out of the structure, and smoke could be seen for miles.
The initial report was that a couch was on fire, but an official cause of the fire has not been determined. About 100 firefighters responded to the fire.
The cause of the fire was under investigation.