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.

ABC News
Firefighters and first responders at the scene of a building explosion in Aliso Viejo, Calif., May 15, 2018.
PHOTO: Firefighters and first responders at the scene of an explosion in Aliso Viejo, Calif., May 15, 2018. ABC News
Firefighters and first responders at the scene of an explosion in Aliso Viejo, Calif., May 15, 2018.
PHOTO: The aftermath of an explosion at a building in Aliso Viejo, California, May 15, 2018.Zubin Ghafari-Saravi
The aftermath of an explosion at a building in Aliso Viejo, California, May 15, 2018.

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.

PHOTO: The scene of an explosion in Aliso Viejo, Calif., is pictured May 15, 2018.Raul Hernandez
The scene of an explosion in Aliso Viejo, Calif., is pictured May 15, 2018.
PHOTO: The aftermath of an explosion at a building in Aliso Viejo, California, May 15, 2018.Zubin Ghafari-Saravi
The aftermath of an explosion at a building in Aliso Viejo, California, May 15, 2018.

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.

PHOTO: Firefighters and sheriffs deputies escort children from Academy on the Hill pre-k school in Aliso Viejo, Calif., May 15, 2018, after a fatal explosion nearby.Mindy Schauer/The Orange County Register/SCNG via AP
Firefighters and sheriff’s deputies escort children from Academy on the Hill pre-k school in Aliso Viejo, Calif., May 15, 2018, after a fatal explosion nearby.
PHOTO: People are evacuated after a building explosion in Aliso Viejo, Calif., May 15, 2018.ABC News
People are evacuated after a building explosion in Aliso Viejo, Calif., May 15, 2018.
PHOTO: Firefighters and first responders at the scene of an explosion in Aliso Viejo, Calif., May 15, 2018.ABC News
Firefighters and first responders at the scene of an explosion in Aliso Viejo, Calif., May 15, 2018.

“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.

PHOTO: The scene of an explosion at an medical building in Aliso Viejo, Calif., is pictured May 15, 2018.ABC News
The scene of an explosion at an medical building in Aliso Viejo, Calif., is pictured May 15, 2018.

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