FBI assistant director in charge of the Los Angeles office Paul Delacourt speaks during a news conference in Orange, Calif. on Monday, March 4, 2019. A California man was arrested in a spa explosion that killed is ex-girlfriend last year after materials similar to those used in the bomb were found in his home and car, federal authorities said Monday. Stephen Beal, was arrested on suspicion of malicious destruction of a building that included a death. The charge carries a possible life sentence. (AP Photo/Amanda Lee Myers)

ORANGE, Calif. - A California man was arrested for a spa bombing that killed his ex-girlfriend last year after a painstaking analysis linked explosive remnants to the suspect, his home and car, federal prosecutors said Monday.

Stephen Beal, 59, had been arrested shortly after the bombing last year, but explosives charges were dropped and he was freed when prosecutors questioned whether material found at his home constituted a "destructive device." Further testing and investigation led to Beal's arrest Sunday on suspicion of malicious destruction of a building that included a death, a charge that can carry a life sentence.

The May 15 bombing killed Ildiko Krajnyak, 48, and seriously wounded two female clients when she opened a box that erupted in a fiery explosion at the spa in the city of Aliso Viejo, about 50 miles (80 kilometres) south of Los Angeles.

"This was a horrific intentional attack that killed an innocent woman and severely injured two others," U.S. Attorney Nick Hanna said at a news conference.

Krajnyak had told friends she was afraid of Beal after he made threats following their breakup. Beal told investigators he felt betrayed when she told him she was in a relationship with another man, Hanna said.

Authorities suspect Beal, a partner in the salon, delivered the bomb to the business while Krajnyak was visiting family in Hungary.

Krajnyak had returned from that trip and had just finished treating a mother and daughter, who were at the front desk when she opened a cardboard box that exploded.

The blast knocked the two customers off their feet and the older woman told police it lit "everything on fire."

First responders thought they were dealing with a car bombing or gas main rupture, said Paul Delacourt, the FBI's assistant director in charge of the Los Angeles office.

The explosion blew out a big hunk of the building and body parts were found in the parking lot.

Investigators found two improvised explosive devices, three firearms and more than 100 pounds (45 kilograms) of explosive material during a search Beal allowed of his Long Beach house following the blast.

Beal, a model rocket hobbyist, denied making bombs and said then he did not have material for an explosion as powerful as the one he saw on television, according to court records.

Over nine months, investigators were able to make the case by piecing together fragments from the bomb they could connect to Beal, including a battery, a wire and boxes similar to the one that contained the deadly bomb.

"While the victim had expressed fears about Mr. Beal and he possessed material that could be used to manufacture a bomb, it was forensic evidence collected at the scene, analyzed and pieced together into a coherent whole that allowed us to file this case," Hanna said.

Delacourt detailed how investigators and personnel at the agency's Quantico, Virginia, lab were able to connect Beal to internal components of a battery found 10 days after the blast.

The battery part was traced to a specific shipment that arrived in the U.S. from China and ended up in a store near Beal's home. Video showed Beal paying cash for that type of battery. Investigators were able to eliminate others who bought those batteries from suspicion.

"This was the exhaustiveness of the effort," Delacourt said.

Beal was ordered detained after a brief appearance in Santa Ana federal court.

He did not enter a plea and is scheduled for arraignment March 25. His lawyer declined to comment.

Outside court, Irene White, a friend of Krajnyak's family, said they continue to grieve her death and hope a conviction brings justice.

In Beal's neighbourhood, Steve Young said he did not think his friend was capable of harming anyone.

"I don't feel that he's the type of person that would do this sort of thing," he said

Beal and Krajnyak split up, but remained business partners at the spa where she was a licensed cosmetologist.

An affidavit in support of the arrest said during a voluntary interview Beal said the romance "cooled" in early 2018.

"Beal stated he took a trip with Krajnyak to Portugal during which Krajnyak admitted to Beal that she was in an intimate relationship with another man from Northern California," the affidavit said "Beal stated he was hurt and betrayed when Krajnyak told him this."

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Melley reported from Los Angeles. Amanda Lee Myers and John Antczak in Los Angeles, and Michael Balsamo in Washington, D.C. contributed to this report.

News from © The Associated Press, 2019