"There's heavy damage on the first floor corner of the building, walls and windows blown out," a fire official said.
LOS ANGELES — An explosion at a Southern California building that killed one person and injured three others may have been an "intentional detonation," law enforcement officials familiar with the investigation said, although other officials cautioned that a cause and whether it was intentional has not been determined.
The investigation into the explosion that occurred in Aliso Viejo at around 1 p.m. local time on Tuesday (4 p.m. ET) is being assisted by the FBI and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, officials said.
Multiple law enforcement officials familiar with the investigation told NBC News that preliminarily, the explosion is believed to have been the result of an "intentional detonation," and that authorities were investigating how the explosive device was sent or delivered to the building. Those sources said it appeared to have been contained in some sort of package.
But Orange County sheriff's Commander Dave Sawyer told reporters Tuesday evening local time that "we cannot say exclusively that this explosion was intentional," adding that "as soon as we can confirm that information, we will share it with you."
The woman that was killed was not identified. That person and the three people who were injured were likely in close proximity to the explosion, Sawyer said. The blast occurred at a suite on the first floor of the building, he said.
"We'd just like everyone to know that our thoughts are with her family," Sawyer said of the person who was killed.
Capt. Tony Bommarito, a public information officer for the Orange County Fire Authority, said the department was called at around 1:09 p.m. on a report of an explosion, and units that arrived "found an obvious explosion, with debris scattered into the parking lot and into the road."
"There's heavy damage on the first floor corner of the building, walls and windows blown out," he said earlier Tuesday evening.
Sawyer said that investigators were going through the interior of the building to find out a cause. He said that any blast of such magnitude would be investigated as suspicious.
"We have not found any type of specific device inside of the building right now that could tell us or lead us to exactly what the device was, if it was a device," he said.
Three people were transported to local hospitals, Bommarito said. He did not characterize their injuries.
"At this time, at this point of investigation, there's nothing that indicates there were any threats made towards this business, or any other types of incidents that occurred before this explosion," Sawyer said earlier Tuesday evening.
An FBI spokesperson said earlier Tuesday that the agency was sending a Joint Terrorism Task Force to assist in the explosion investigation as a precaution.
The building where the blast occurred is across the street from an academy and day care facility, NBC Los Angeles reported. The children were reunited with their parents at a nearby Target parking lot.
Tony Dik, whose son attends the preschool, told the station that it was emotional for parents who realized that their children were on the same street 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," his son, 6-year-old Kingston, said, according to the station.
Aliso Viejo is south of Los Angeles.