An explosion that rocked a Southern California medical office building on Tuesday, killing a woman and injuring three other people, is being investigated as intentional and investigators believe the blast may have come from a package, a U.S. official told The Associated Press.

The explosion at the office building in Aliso Viejo — about 50 miles (80.5 kilometers) south of Los Angeles — blew siding off the walls, exposing insulation and framing.

Authorities had not found an explosive device Tuesday night but were searching through debris that was left after the blast. The official said the information was based on a preliminary investigation.

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.

Orange County sheriff's Cmdr. Dave Sawyer said the size of the explosion made is suspicious.

"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," Sawyer said.

Bomb technicians from the Orange County Sheriff's Department, the FBI and other agencies were investigating, said Paul Delacourt, the assistant director in charge of the FBI's field office in Los Angeles. He referred to the explosion site as a "crime scene."

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.

Orange County Fire Authority Chief Brian 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.