Accused Gilgo Beach serial killer Rex Heuermann allegedly had “an arsenal” of up to 300 guns stashed in a vault in the basement of his Long Island home — or three times as many weapons as first believed.

The 59-year-old suspect’s massive collection of semi-automatic rifles, revolvers and pistols was found in a container that was walled off behind a locked metal door, CNN reported Monday, quoting a source familiar with the case.

Cops had said Heuermann, an architect and married father of two, had 92 gun permits. He was an avid hunter, CNN said.

But police have since found a total of between 200 and 300 guns at his family’s Massapequa Park home, the outlet said.

“He had an arsenal in a vault that he had downstairs,” Suffolk County Police Commissioner Rodney Harrison told Fox News earlier Monday. “It’s concerning, regarding the guns being registered or legal or not. That’s something we’re still taking a look at.

“Anytime somebody has that type of arsenal, we have some concerns,” Harrison added.

Police have been combing through the home since Heuermann’s arrest Thursday in the deaths of three women whose bodies were among 11 found in Gilgo Beach over the past decade or so. He is the prime suspect in the death of a fourth Gilgo Beach victim, too, cops have said.

As part of the case, investigators also have targeted a storage unit he maintained in Amityville.

Who were the Gilgo Beach victims?

Suspected serial killer Rex Heuermann — a New York City architect and married dad of two — was arrested in connection with the long-unsolved Gilgo Beach murders. The arrest is tied to the so-called “Gilgo Four,” women found wrapped in burlap within days of each other in late 2010.

The years-long investigation that led to the arrest revolved around the discovery of more than 10 sets of human remains along Ocean Parkway near Gilgo Beach in Suffolk County between December 2010 and April 2011.

Most victims were petite female sex workers with green or hazel eyes. But there were also two exceptions: a 2-year-old girl and a young Asian man.

Melissa Barthelemy, 24

• Barthelemy was a sex worker who lived in the Unionport section of the Bronx and dreamed of one day opening her own beauty salon. She was last seen alive in her basement apartment on Underhill Avenue on July 12, 2009. Heuermann was charged for Barthelemy’s murder in July 2023.

Maureen Brainard-Barnes, 25

• Brainard-Barnes was living in Norwich, Connecticut. She went missing after taking an Amtrak train from New London, Connecticut, to Grand Central Terminal in Manhattan on July 6, 2007. Her remains were found in December 2010. Heuermann was charged for Brainard-Barnes’ murder in January 2024.

Amber Lynn Costello, 27

• Costello, 27, was a sex worker and heroin addict who lived in West Babylon, New York, at a home with a woman and two men. She advertised on Craigslist and Backpage to support her and her roommates’ drug habits. Costello was found on December 13, 2010, after having been last seen leaving her home September 2, 2010. Heuermann was charged for Costello’s murder in July 2023.

Megan Waterman, 22

• Waterman, a 22-year-old mom of one, was last seen on June 6, 2010. She lived in Scarborough, Maine, and earned a living as an escort. She was last seen by her family boarding a New York-bound Concord Trailways bus in Maine. Her body was found on December 13, 2010, on the north side of Ocean Parkway, near Gilgo Beach. Heuermann was charged for Waterman’s murder in July 2023.

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Jessica Taylor, 20

• Remains belonging to Jessica Taylor, a 20-year-old woman working as an escort in New York City, were found in a wooded area in Manorville on July 26, 2003. Her additional remains — initially labeled “Jane Doe No. 5” — were discovered on March 29, 2011, along Ocean Parkway.

Valerie Mack, 24

• Valerie Mack was 24 years old and living in Philadelphia when she went missing. She worked as an escort, using the alias “Melissa Taylor.” Relatives last saw Mack in the spring or summer of 2000 in Port Republic, New Jersey, but she was never reported as missing to the police. Her partial skeletal remains were found in Manorville in September 2000 but were initially known as “Jane Doe No. 6.” More bones were found on April 4, 2011, along Ocean Parkway.

Unidentified Asian man

• The skeletal remains of a yet-to-be-identified Asian man were found along Ocean Parkway on April 4, 2011. It is estimated that the man was between 17 and 23 years old at the time of his death. He was approximately 5 feet 6 inches tall with bad teeth.

‘Peaches’ and her daughter

• An African American woman’s partial remains were discovered in Hempstead Lake State Park back in 1997, and she had become known as “Peaches” because of a bitten tattoo of a peach on her left breast. On April 4, 2011, police uncovered the remains of a toddler, who was about 2 years old at the time of her death. DNA testing confirmed that one of the skeletons was that of the 2-year-old girl’s mother, “Peaches.”

Karen Vergata

• A victim previously referred to as Jane Doe No. 7 has been identified as 34-year-old Manhattan woman Karen Vergata. Vergata is believed to have disappeared around Feb. 14, 1996; two months later, her legs were found in a plastic bag at a park near Fire Island’s Blue Point Beach. At the time of her disappearance, Vergata was believed to have been working as an escort. Two sets of Vergata’s remains were identified in August 2023.

Shannan Gilbert, 23

• Gilbert was a Craigslist escort who lived in Jersey City, traveled with her driver Michael Pak from Manhattan to meet a client, Joseph Brewer, at his home in the Oak Beach Association on the morning of May 1, 2010. She spoke with two neighbors before disappearing. Her body was discovered in a marsh near Oak Beach — about half a mile from where she was last seen alive — on December 13, 2011.

Jessica Taylor, 20

• Taylor, a 20-year-old woman working as an escort in New York City, were found in a wooded area in Manorville on July 26, 2003. Her additional remains — initially labeled “Jane Doe No. 5” — were discovered on March 29, 2011, along Ocean Parkway.

Sandra Costilla

• Costilla was murdered in 1993 but had not been included among the so-called Gilgo Beach victims — until now. Investigators suspected convicted serial killer John Bittrolff in Costilla’s death, but he was never charged in her slaying — which remains one of several unsolved Long Island murders.

They said Monday that they’re focused on finding body parts or “trophies” from the crimes as they continue to scour for evidence.

Cops hauled out a bin filled with rifles from Heuermann’s house over the weekend.

The hulking accused killer is charged with murder in the 2010 slayings of Melissa Barthelemy, 24, Megan Waterman, 22, and Amber Lynn Costello, 27, whose remains were found in a marshy stretch along the South Shore not far from Heuermann’s home. Authorities said he is the prime suspect in the death of Maureen Brainard-Barnes, 25, also.

Harrison, a former NYPD chief, reopened the case when he took over as Suffolk’s top cop at the start of last year.

“We found that he was an architect and some other things, and we were able to do some phone records and some other background checks and how he uses his credit cars and look at his family,” the police chief said of Heuermann.

“He has a wife and two kids and just his lifestyle, and we started getting closer and closer and saying, ‘Hey, this may be our person,’ ” Harrison said.

Police ultimately linked Heuermann to the crimes through tips from witnesses — including the pimp of one of the victims — and DNA pulled from a discarded pizza crust that tied him to the crime scene.