Late Tuesday afternoon, NBC4 - citing unnamed federal sources - said investigators had determined that the blast was an "intentional detonation", and the explosive device was delivered to the building, addressed to a specific person.
The explosion ripped through the first floor of the two-story medical building, blowing out walls and windows - and sending the preschool kids scattering, along with their parents. Investigators believe the blast may have come from a package, according to an official who was briefed on the investigation but wasn't authorized to discuss the matter publicly and spoke to the Associated Press on condition of anonymity. Investigators said they hadn't yet located an explosive device and were searching through debris.
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 fatal explosion nearby.
An Orange County Sheriff's Department spokeswoman said: "We're trying to determine if a vehicle was involved or if it was an accident or inside the building".
Firefighters put out a small fire and found one woman dead inside the building, the authorities said.
Bomb technicians from the Orange County Sheriff's Department, the Federal Bureau of Investigation and other agencies were investigating, said Paul Delacourt of the Federal Bureau of Investigation.
A woman who was on her way to an appointment in the area told CBS2 she arrived just after the blast occurred, and she saw fire on the roof of the building.
McWilliams said she saw two burned women staggering out of the building, covered in ash and soot.
Children at a day care across the street from the building were evacuated.
They said they were not actively looking for any suspects.
There were unconfirmed initial witness reports that the blast may have been a gas explosion, but Bommarito said it is rare for such gas-fueled blast to occur in a commercial building. The other woman was bleeding from her head.
"The survivors' injuries were consistent with an explosion, but not necessarily consistent with a bomb".