Deadly explosion at California day spa wasn't an accident, officials say

The mysterious, deadly explosion that rocked a Southern California day spa on Tuesday doesn't appear to have been an accident, the Orange County Sheriff's Office and the FBI said today.

The powerful explosion in Aliso Viejo, which is about 50 miles south of Los Angeles, left one woman -- believed to be the day spa owner -- dead and three others injured.

The motive is not clear, officials said today, adding that no arrests have been made.

The working theory is that the explosion was from a device, the FBI said. Two law enforcement sources briefed on the investigation told ABC News that preliminary indications are that the device was inside a package that was delivered to the business.

The blast sent plumes of white smoke into the air and debris flying into the parking lot.

Witness Dong Shin told ABC News he heard a booming sound and then "felt the ground shake."

When he saw fire and debris, Shin said, his first thought was a bomb.

Children at a day care facility across the street from the building were evacuated. Some were seen walking outside holding hands, while others were wheeled out in cribs.

No children were hurt, officials said.

"It's horrible, it's a tragedy,” Orange County Fire Authority spokesman Tony Bommarito told ABC News. “Anytime you're near this or you see it, it's going to affect you.”

The scene has been rendered safe, the city said.

ABC News' Kayna Whitworth and Jenna Harrison contributed to this report.