ALISO VIEJO, Calif. (AP) — The Latest on an explosion that rocked a Southern California office building (all times local):

9 p.m.

A U.S. official says a deadly explosion at a Southern California office building is being investigated as an intentional act and investigators believe the blast may have come from a package.

One woman was killed and three others were injured during the Tuesday afternoon explosion at a medical office building in Aliso Viejo.

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 official was briefed on the investigation but wasn't authorized to discuss the matter publicly and spoke to The Associated Press on condition of anonymity.

Agents from the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives were at the scene Tuesday night.

Sheriff's officials said the size of blast made it suspicious.

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Associated Press writer Michael Balsamo in Los Angeles contributed to this report.

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8:20 p.m.

Authorities say explosives experts are trying to determine what caused an explosion that killed a woman and injured three other people at a Southern California medical office building.

The Tuesday afternoon blast on the first floor of an Aliso Viejo building in Orange County shattered windows and forced children in a nearby daycare center to be evacuated.

Mary McWilliams of San Clemente tells the Orange County Register that she saw two women with burns stagger out of the building. Fire officials say a third person suffered smoke inhalation.

Orange County sheriff's experts along with FBI, the ATF and other federal agencies are investigating the cause of the blast. Orange County Sheriff's Cmdr. Dave Sawyer said Tuesday night that the size of the explosion makes it suspicious.

However, he says no explosive device has been found in the building and investigators still aren't saying whether the explosion was accidental or intentional.

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7:10 p.m.

A witness describes seeing two women with burns following an explosion that killed one person and injured three at a Southern California medical office building.

Mary McWilliams tells the Orange County Register that she arrived for an appointment Tuesday in Aliso Viejo to find smoke surrounding the area and car alarms going off.

McWilliams says she saw two women covered in ash and soot stagger out of the building. McWilliams says one woman had skin peeling on her arm. The other woman was burned and her head was bleeding.

The injured were taken to a hospital and there's no immediate word on their conditions.

Federal, state and local authorities, including the FBI and the Department of Homeland Security, are investigating the blast, which occurred on the first floor of a building that was under renovation.

Orange County Sheriff's Cmdr. Dave Sawyer says authorities haven't determined whether the explosion was accidental or intentional.

Officials haven't released information about the person who was killed.

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2:47 p.m.

Authorities say one person is dead and three others have injuries after an explosion rocked a Southern California office building.

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

Fennessey says the blast involved a building under renovation. The cause of the blast is being investigated.

There was no immediate information about the person who was killed. The injured were taken to a hospital.

Fennessy says children at a daycare across the street were not injured. They were being reunited with parents at a nearby store parking lot.

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2:37 p.m.

Authorities say one person is dead and two others have injuries after an explosion rocked a Southern California office building.

Orange County Fire Authority Chief Brian Fennessy says 10 engines and 70 firefighters were dispatched in response to a 1:09 p.m. Tuesday call from Aliso Viejo.

Fennessy says the blast involved a building under renovation. The cause of the blast was being investigated.

There was no immediate information about the person who was killed. The injured were taken to a hospital.

Fennessey says children at a daycare across the street were not injured. They were being reunited with parents at a nearby store parking lot.