ALISO VIEJO (CNS) - Investigators from the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives assisted Orange County Sheriff's detectives this morning at the scene of an explosion in a medical building in Aliso Viejo, where a woman was killed and three other people were injured.
Several media outlets, citing unnamed law enforcement sources, said the explosion Tuesday afternoon was intentional and may have come from a device that was delivered to the building at 11 Mareblue.
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.
Tuesday night, the L-A Times cited two law enforcement sources who said ``the explosion was likely caused by a package that was delivered to the location, but cautioned that it would be speculative to say the blast was intentional.''
Orange County Sheriff's Cmdr. Dave Sawyer told reporters on Tuesday that investigators ``don't know if it was an intentional detonation of a device.'' He said investigators had not found an explosive device at the scene on a lower floor of the building.
The explosion sparked a fire in the building about 1:10 p.m. Tuesday, but it was quickly doused after a large-scale response by fire and sheriff's officials.
Authorities initially indicated a car had crashed into the building, but Orange County Fire Authority Capt. Tony Bommarito and sheriff's officials said later that was not the case.
``We have confirmed there was no vehicle inside,'' Bommarito said, adding there was an explosion inside the building. The woman who died was inside the building when the blast occurred.
Although the cause of the blast was still unknown, Bommarito told reporters Tuesday afternoon that there was ``no immediate hazard'' to the community.
Authorities did not actually enter the building until late Tuesday, with a bomb squad sifting through the wreckage inside the structure. Outside the building, a massive contingent of sheriff's deputies and fire crews amassed, joined by federal agencies summoned to assist with the probe.
Sheriff's deputies snapped photos of vehicles that were leaving the scene throughout the afternoon.
Bommarito said one of the injured people suffered from smoke inhalation, but the extent of the other victims' injuries was unknown.
Children and staff at nearby Academy on the Hills preschool initially sheltered in place while the investigation continued, but children were later evacuated to a Target store parking lot so they could be reunited with their parents. No children were injured, officials said.
Video from the scene showed teachers leading the children along a sidewalk toward the Target. Other people could be seen wheeling cribs down the street as the area was evacuated.
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.
``Then everyone started running out of buildings everywhere with looks on their faces of just horror,'' the woman said. ``... I saw two women come out full of blood, hair singed, just glass stuck to the hair, glass stuck to their bodies. Their skin was burned and peeled back, and they were just in shock.
``... They weren't on fire, they were helping each other across the street, walking very slowly and a lot of shock, shaking, just full of ash all over their face, eyes bloodshot, just blood all over,'' the woman said.
Sheriff's deputies were called to the same block of Mareblu at 11:35 p.m. Monday for a ``patrol check,'' according to sheriff's records. It was not immediately clear what prompted that call.