ALISO VIEJO, Calif. — An explosion rocked a Southern California medical office building Tuesday, killing a woman and injuring three other people, authorities said.

Orange County Fire Authority Chief Brian Fennessy said 70 firefighters were dispatched in response to a 1:09 p.m. Tuesday call from Aliso Viejo.

The blast involved a building under renovation but the cause was not immediately known, Fennessy said.

The explosion blew siding off the walls, exposing insulation and framing. Federal, state and local investigators were on the scene.

"The corner of that building, the whole bottom floor is pretty much blown out," fire Capt. Tony Bommarito said.

Bomb technicians from the Orange County Sheriff's Department, the FBI and other agencies were investigating, said Paul Delacourt of the FBI. He referred to the explosive scene as a "crime scene."

However, sheriff's Cmdr. Dave Sawyer said the size of the explosion made it suspicious but no explosive device had been found and it was unclear whether the blast was accidental or intentional.

Mary McWilliams told the Orange County Register that she arrived for an appointment to find smoke surrounding the area and car alarms going off.

McWilliams said she saw two burned women staggering out of the building, covered in ash and soot.

One woman, who had skin peeling on her arm from burns, said, "Take care of my mother," McWilliams said. The other woman was bleeding from her head.

"I thought it was a little fire until I saw everyone's faces and reactions," McWilliams said.

A third person suffered smoke inhalation, fire officials said.

Fennessy said children at a daycare across the street were not injured.

Little children holding hands and others being pushed in rolling cribs were led by firefighters to a Target parking lot to be united with parents.