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A Southern California man has been charged in connection with a package bomb explosion in 2018 that killed his ex-girlfriend and injured two others at a beauty spa, a federal prosecutor said Monday.

Stephen William Beal, 59, was charged with one count of "malicious destruction of a building resulting in death," after a May 15 explosion at Magyar Kozmetica spa in Aliso Viejo, Calif., killed the owner, Ildiko Krajnyak, 48.

Investigators said they believe Krajnyak was Beal's intended target, said U.S. Attorney for the Central District of California Nicola T. Hanna in a press conference on Monday.

"This was a horrific and intentional attack," Hanna said.

"The citizens of Orange County have not seen an explosion this large in 30 years,” Paul Delacourt, assistant director in charge of the FBI's Los Angeles field office, said at the press conference.

Beal told investigators that in the months leading up to the incident, Krajnyak revealed to him she was seeing another man.

He said this left him "hurt" and "betrayed," according to the prosecutor.

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Krajnyak, a mother and cosmetologist, had just returned from visiting family in her native Hungary when she received a package at work.

Two women who were frequent clients of the spa, were present at the time, the affidavit said.

When they were about to pay for their services shortly before 1 p.m., one of the victims said she saw Krajnyak open a cardboard box, according to the federal criminal complaint.

"Victim #1 saw it explode and recalled being blown backward by the explosion onto the floor," the complaint said. "She also saw flames and smoke."

Krajnyak was killed instantly, and the blast was so powerful her remains were found outside of the building, authorities said. The other two women were injured.

FBI agents said that during a search at Beal's home in Long Beach, Calif., they found bomb-making components that had "no meaningful difference" with the material found at the scene of the explosion.

Investigators determined that Beal purchased a battery consistent with the partially destroyed battery found at the blast scene at a store near his home one week prior to the explosion, the complaint said. And, eight days before the incident, he purchased three cardboard boxes very similar to the box the deceased victim was opening when the bomb detonated.

The agent also said that chemicals detected at the crime scene were also found in Beal's car, which he purchased only three months before the incident. Beal told investigators he was one of few people who had keys to the day spa offices.

Beal was initially arrested and charged with possessing an unregistered destructive device when federal investigators searched his home days after the explosion.

During the search, approximately 130 pounds of explosive-related materials including electric matches and wire were discovered, which Beal said were supplies for constructing model rockets.

Initial charges against him were dropped shortly after the FBI said it had questions about whether material found in the house met the legal definition of a "destructive device."

Several law enforcement sources told NBC News in recent months Beal remained the focus of the bombing investigation, although he was not named publicly as a suspect.

Authorities said Monday that if convicted, Beal faces potential life in prison without the possibility of parole.