One woman was killed and three others were injured during the Tuesday afternoon explosion at a medical office building in Aliso Viejo.
A USA 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.
There are unconfirmed reports of explosive devices on the scene of the blast in Orange County.
Investigators are considering the incident as an intentional act, due to the size of the explosion.
The FBI was sending resources to the scene, but agency spokesperson Mike Gifford said there was no early indication of terrorism, CNN reporter Kristen Holmes tweeted.
There was a daycare located across the street from the building with the explosion, but officials confirmed no children were injured and they were working to reunite the kids with their families.
He said the person who died was inside the building when the blast occurred.
McWilliams said she saw two burned women staggering out of the building, covered in ash and soot.
Fire authority spokesperson Captain Tony Bommarito later updated that three others suffered injuries and were taken to the hospital.
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.
Bomb squad investigators and the Federal Bureau of Investigation worked to determine the cause of the blast, but authorities said there was no ongoing threat to the community.
Fire and sheriff's officials said the building appeared to be undergoing some type of renovation.
There was no word on whether any explosive devices were in the building.