Spa owner Ildiko Krajnyak was opening a package that had piled up with mail during her recent trip to her native Hungary when it exploded.
News reports of the blast quickly reached Stephen Beal, her ex-boyfriend and a partner in the Southern California business.
At the urging of his new girlfriend, Beal phoned police and then let them search his house. They found more than 100 pounds (45 kilograms) of explosive material and charged him Thursday with possessing an unregistered destructive device.
While not charged with the fatal explosion, the arrest puts Beal in custody as authorities investigate what they believe was a targeted bombing.
Beal, a model rocket hobbyist, told investigators he had not made any bombs and did not have material for an explosion as powerful as the one he saw in news coverage.
Beal, 59, did not enter a plea during his initial appearance in U.S. District Court in Santa Ana and his case was continued to Monday. His public defender refused to comment.
The criminal complaint was accompanied by an affidavit from an FBI special agent that briefly described the relationship between Beal and Krajnyak, as well as the grisly blast.
The two had met online in June 2016 and dated about a year and a half. Photos on his Facebook page showed them living it up in far-flung places: a beach in Cuba, restaurants in Portugal, and riding jet skis in Mexico.
The romance cooled earlier this year after disputes over exclusivity and finances, FBI Special Agent Evan Jesch wrote.
They remained business partners, with Beal serving as spa manager, and the two operated a separate cosmetics firm. State records show Beal and Krajnyak were officers in a skin care business called I&S Enterprises.
Beal was listed on the spa's lease because Krajnyak and her estranged husband had filed for bankruptcy, he said. Beal paid the $1,500 monthly rent and half the spa's operating costs. He had to loan Krajnyak money some months to cover her costs.
Beal was once an investment banker who turned to acting after his wife's death several years ago, said friend and neighbor Steven Young. Beal appeared in several small films.
"You can't imagine him doing what we've been hearing," Young said, referring to the blast. "I don't recall him ever being visibly angry at anything. He's been a good neighbor and a good friend."
Nathan Beal said it was "highly unlikely" his father was guilty of any charge related to a destructive device.
The father and son build sport rockets fuelled by a propellant commercially available at hobby stores. The biggest of the rockets used about 5 pounds (2.3 kilograms) of fuel in multiple motors.
Stephen Beal told authorities he hadn't been involved with rockets since 2004. He said he had once made fireworks and mortars, but quit that after Sept. 11 because he said he "did not want to give the wrong impression."
When confronted with photos of the IEDs recovered, Beal initially denied recognizing them but later explained exactly where they were found.
"Beal also claimed that he had not made any bombs or anything that would catch on or cause fire," Jesch wrote.
Beal's wife died in 2008 and his adult children had moved out of the house, Young said. Beal seemed happier after meeting Krajnyak and spoke of her frequently.
"I think he was happy to have found someone he could connect with," said Young, who never met Krajnyak.
Krajnyak, 48, a cosmetologist, had just returned to California after visiting family in her native country.
Laszlo Krajnyak choked up while speaking from his home in Tiszaujvaros about his sister's recent visit. Last month, he had come to the U.S. to help with her business that offered services such as facials, waxing and wrinkle treatments.
Friends and neighbors created an online campaign to pay for Krajnyak's funeral and to fly her son home from Seattle where he attends college.
Her son, Keanu Vestil, 20, arrived home Thursday afternoon and said he was overwhelmed by the support.
"All of this is still very raw," he said. "There are just so many things we don't know yet."
The spa in Aliso Viejo, about 50 miles (81 kilometers) south of Los Angeles, had just reopened when the blast rocked the two-story office building. Krajnyak was blown out of the building in the fiery explosion that shook the surrounding area.
A mother and daughter from Hungary who Krajnyak had just treated were the only other people in the spa at the time.
The younger woman said she was at the counter preparing to pay when Krajnyak picked up one of about three or four brown cardboard boxes on the floor and opened it.
The blast knocked her and her mother off their feet. Her mother told police she saw "everything on fire."
Both women, who were not named, were seriously injured and had to undergo surgery.
Bomb technicians found a 9-volt battery, a cellphone and loose wires at the origin of the blast.
One official briefed on the investigation told The Associated Press that Krajnyak was the intended recipient of an explosive package. The official was not authorized to disclose details about the investigation and spoke on condition of anonymity.
Balsamo reported from Los Angeles. Associated Press journalists Amy Taxin in Orange, John Antczak and Brian Melley in Los Angeles, Kester Eddy in Budapest, Hungary, and researcher Rhonda Shafner in New York contributed to this report.Copyright 2018 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.