Aliso Viejo Explosion: Everything We Know About 'Intentional Detonation' in California

At least one person was killed and three others injured as an explosion and fire tore through a medical building in Aliso Viejo, Orange County, at around 1.10 p.m. on Tuesday afternoon.

Officials said the incident appears to have been an "intentional detonation" and the device may have been delivered as a package to a specific person, reported NBC4.

After the explosion, bomb squad investigators arrived on the scene to search for more dangerous devices, while children at a nearby day care were evacuated. The preschool was not affected by the blast.

At least one person was killed and three others injured after an explosion ripped through a medical building in Orange County on Tuesday afternoon. OC Sheriff, CA Twitter

Tony Dik, whose child attended the day care, told NBC that parents were worried and emotional when they found out that the preschool was on the same road as the explosion. "When I was playing outside I heard the big crash. I thought it was a garbage truck but it was a building that smashed," said Kingston, Dik's 6-year-old son.

According to the Orange County Fire Authority, the blast originated from the first floor of the building.

Witness Mary McWilliams told NBC-4 that she saw several women, covered in blood and ash, running away from the building. One of them reportedly said "take care of my mother, take care of my mother."

Fire fighters arrived at the site shortly after the incident to extinguish a blaze that was caused by the explosion. Although investigators and FBI are still determining the cause of the blast, officials have announced the threat is over.

Initial reports claimed a car crash caused the explosion inside the facility. Soon after, Carrie Braun, an Orange County Sheriff's Department spokesperson, denied that a car was involved in the incident and said the investigation into what caused the explosion is still ongoing. Officials also said in earlier coverage that no device was seen in the building.

"We have not found any type of specific device inside of the building right now that could tell us or lead us to what exactly the device was, if it was a device," Cdr. Dave Sawyer, from the Orange County Sheriff's Dept., said at a press conference on Tuesday evening.

The victim, who was confirmed dead on the scene, has not yet been named.