Man arrested for allegedly having an unregistered destructive device amid investigation into deadly explosion

ABC News(LOS ANGELES) -- A Southern California man has been arrested on suspicion of having an unregistered destructive device amid the investigation into a deadly explosion at a day spa, the FBI said.

But Stephen Beal, 59, has not been charged in connection with the Tuesday explosion that killed a day-spa owner at her Aliso Viejo, California, spa, federal authorities cautioned.

The explosion doesn't appear to have been an accident, the FBI said Wednesday.

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

The motive wasn't clear, the FBI said Wednesday.

During the investigation into the explosion, officials served a search warrant at Beal's Long Beach, California, home and found the destructive device, the FBI said today.

Beal is expected to make his first appearance in U.S. District Court in Santa Ana this afternoon.

The FBI did not immediately provide additional information.

The Tuesday 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.

The scene has been rendered safe.

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