By Darran Simon, Sonya Hamasaki, Sara Sidner and Eliott C. McLaughlin

    (CNN) -- A deadly explosion at a California day spa that killed the business owner, was likely caused by a device and was not accidental, authorities said Wednesday.

The explosion Tuesday blew out walls and windows of the facility, heavily damaging the business, which was on the first floor of a two-story medical office building in Aliso Viejo, about 50 miles southeast of Los Angeles, according to Orange County officials. The powerful blast injured three people and caused the ground to shake in nearby structures.

Evidence shows the materials believed to be involved in the explosion were not consistent with items used at a day spa, Orange County Undersheriff Don Barnes said. Authorities are treating the explosion as a crime, but are still working to confirm if the blast was intentional. No arrests have been made.

Earlier, a law enforcement source told CNN that the explosion may have been detonated on purpose, but the source also cautioned that the investigation was in its preliminary stages.

"At this point, we do not believe this was an accident. We've ruled out a gas main explosion and other potential theories," said Paul Delacourt, assistant director in charge of the FBI's Los Angeles field office. "At this point, our working theory is that this explosion was caused by a device."

Delacourt said it is "premature to speculate on the nature of the components" of the device.

Authorities are still trying to determine how the device arrived at the location, Delacourt said. He said there is no indication there is a second device.

Barnes said investigators don't know the motive at this point.

Barnes said authorities believe the victim who was killed in the blast was Ildiko Krajnyak of Trabuco Canyon. Public records show that Krajnyak was 48; authorities said she was 40.

A voicemail message for the spa, Magyar Kozmetika, said Tuesday would be Krajnyak's first day back from a trip.

Two women, believed to be patrons of the day spa, are undergoing surgery and are expected to survive. A third woman suffered smoke inhalation and was treated and released at the scene.

Authorities served three search warrants Wednesday -- one at the Aliso Viejo business, one in Long Beach and one in Trabuco Canyon, Barnes said.

Explosion felt 'like a big earthquake'

Dong Shin, a witness, said he heard a loud boom minutes before his Tuesday appointment in a nearby building.

"And the ground shook," he said. "While the ground was shaking ... my body jolted and my head hit the wall."

Shin said it felt like "a big earthquake," and as he and others rushed downstairs to get out of the nearby building, he saw two holes in the wall of the medical facility. He also saw "fire, smoke, insulation popping out of the walls, a lot of scared people running around, a lot of commotion," he said.

One woman who ran out of the medical facility had her face covered in blood, Shin said.

The blast, which erupted around 1 p.m (4 p.m. ET), caused extensive damage to buildings in the area, Orange County Sheriff's Department Commander Dave Sawyer said earlier.

"Anytime you see an explosion of this magnitude, it would definitely be suspicious to us, and that's why we rolled out all the resources to get to the bottom of it," Sawyer said earlier.

Nothing indicates there were any threats made before the blast, Sawyer said.

There was no initial indication of terrorism, FBI spokesman Mike Gifford said.

The FBI, the Department of Homeland Security and the Bureau of Alcohol, Tobacco, Firearms and Explosives are assisting investigators.

The-CNN-Wire

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