The Latest: Authorities investigate California office blast

In this photo taken from video provided by KABC-TV, children are evacuated from a preschool across the street from a building that was rocked by an ex

In this photo taken from video provided by KABC-TV, children are evacuated from a preschool across the street from a building that was rocked by an ex

Firefighters and sheriff's deputies escort children from Academy on the Hill pre-k school in Aliso Viejo, Calif., on Tuesday, May 15, 2018, after a fa

Firefighters and sheriff's deputies escort children from Academy on the Hill pre-k school in Aliso Viejo, Calif., on Tuesday, May 15, 2018, after a fa

This photo taken from video provided by KABC-TV shows a building after an explosion rocked it in Aliso Viejo, Calif., Tuesday afternoon, May 15, 2018.

This photo taken from video provided by KABC-TV shows a building after an explosion rocked it in Aliso Viejo, Calif., Tuesday afternoon, May 15, 2018.

This image taken from cellphone video shows a building after a fatal explosion in Aliso Viejo, Calif., Tuesday, May 15, 2018. The cause of the blast i

This image taken from cellphone video shows a building after a fatal explosion in Aliso Viejo, Calif., Tuesday, May 15, 2018. The cause of the blast i

LOS ANGELES (AP) — The Latest on a deadly explosion at a Southern California office building (all times local):

8 a.m.

Officials are investigating an explosion that killed a woman and injured three other people at a Southern California medical office building.

Orange County Sheriff's Department spokeswoman Carrie Braun says investigators worked through the night and were still at the scene Wednesday morning.

A U.S. official told The Associated Press that the explosion is believed to have been intentional and may have come from a package.

The official was briefed on the investigation, was not authorized to discuss the matter publicly and spoke to The Associated Press on condition of anonymity.

A sheriff's official said late Tuesday that no specific device had been found.

— Associated Press Writer Michael Balsamo

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11:08 p.m.

Investigators are searching for clues about an explosion at a Southern California medical office that left a woman dead and three other people injured.

The Tuesday afternoon blast occurred in Aliso Viejo, which is about 50 miles south of Los Angeles. It blew siding off the walls, exposing insulation and framing and shattering windows at the two-story building.

A federal official told The Associated Press late Tuesday that the explosion is believed to have been intentional and may have come from a package.

The official was briefed on the investigation, was not authorized to discuss the matter publicly and spoke to The Associated Press on condition of anonymity.

Authorities say the dead woman had been inside the building.

Witness Mary McWilliams tells the Orange County Register that she saw two burned women stagger from the building. Fire officials say a third victim had smoke inhalation.

There's no word yet on a motive for the explosion.

— Associated Press Writer Michael Balsamo.