A Norristown man on Tuesday pleaded guilty to raping a woman in a brazen, broad-daylight assault in a public park four years ago.

Mason Alexander Hall, 21, admitted his guilt in the rape, along with charges of possession of an instrument of a crime, reckless endangerment, unlawful restraint and related offenses during a hearing at the county jail, six miles from the site of the attack.

Hall said little during the hearing before Montgomery County Court Judge Thomas Branca, only answering the questions asked by his attorney, Allan Sodomsky. The 21-year-old entered an open guilty plea, meaning there was no agreement with prosecutors on the sentence that will be imposed in the case.

In August 2017, the victim, then 19, was walking in Norristown Farm Park when a man grabbed her from behind, pointed a black, semiautomatic gun at her head and threatened to shoot if she tried to flee, investigators said. The attacker forced the woman to a secluded, wooded area, sexually assaulted her and then fled, all without the victim getting a good look at his face.

» READ MORE: Norristown Farm Park sexual assault reward of $10K offered

The crime went unsolved for years, confounding authorities, even as they offered a $10,000 reward for information in the case.

The breakthrough that led to Hall’s arrest in 2019 came from a piece of evidence that Montgomery County District Attorney Kevin Steele said they had from the first day of the investigation: The attacker’s DNA.

Parabon NanoLabs, a Virginia-based company that helped investigators arrest the “Golden State Killer,” created an extensive DNA profile that investigators were able to compare to local and national databases. From that profile, prosecutors narrowed their search to two suspects, including Hall.

The final, determining step came from another piece of DNA, collected a few months after the attack.

» READ MORE: DNA links Montgomery County teen to violent 2017 rape in Norristown Farm Park

During a dispute in Norristown, Hall, then 17, smashed the taillight of a parked car with a hammer, cutting his hand in the process. Norristown Police seized the hammer as part of their investigation into the vandalism, and it sat in the department’s evidence room for years.

After Hall became a suspect in the rape, prosecutors compared the DNA sample from that assault with the sample of Hall’s blood from the hammer. It was a match.

He is scheduled to be sentenced in September.