Gilgo Beach suspect Rex Heuermann may have killed victim in soundproof room, cops using cadaver dogs to search yard

Cops allegedly found a soundproof room in the accused Gilgo Beach killer's basement and think at least one victim was killed there, it surfaced Sunday — as police used cadaver dogs and ground-penetrating sonar to scour his back yard.

A former coworker told The Post that suspected serial murderer Rex Heuermann, 59, once took time off from his job as a Manhattan architect to install a concrete-lined vault at his family's Long Island house.

It's not clear if that basement compartment is the one cops suspect could have been a killing chamber or whether it is possibly the space previously reported to hold Heuermann's up to 300 guns.

But "it's not just a hidden room — it's a serious vault," the ex-colleague said. "It had a huge heavy-duty safe door. He went and poured new concrete walls, massive amount of concrete to encase this room.

"It was maybe 2 or 3 feet thick," the former coworker said.

New York State troopers and Suffolk County investigators have been searching Heuermann's home for the 11 days since the hulking married dad of two was charged in the deaths of three women and named the prime suspect in a slaying of a fourth — all part of the infamous 13-year-old "Gilgo Four" murders.

"This guy is a wacko," said Robert Musto, 64, a retired Long Island Rail Road worker and longtime Massapequa Park neighbor, to The Post on Sunday, referring to Heuermann.

"He's got a soundproof room in his basement," Musto said he was told by cops at the scene. "What do you think that was for?

"They're saying there's evidence he killed at least one of the girls down there," the Long Island neighbor said. "The cops are going to dig all that out. Said they're focused on the soundproof room in the basement but they're going to look at everything."

A law-enforcement source told The Post on Sunday, "Cops are looking for evidence if the victims were in the house, nothing yet. "

Outside the house Sunday, an investigator was seen meticulously moving across ground in the back yard Sunday with a sonar device resembling a lawn mower, video footage showed.

Three cadaver dogs also were brought to the property, as was a backhoe and dump truck for what state troopers at the scene called a "major excavation."

The construction rig was rolled onto the yard shortly before 4 p.m.

Local Kathy Huber, who said she went to high school with Heuermann, said neighbors are OK with the disruption.

"We don't care how long this has to be here," Huber, 57, said Sunday. This is a big community of cops and firemen, and I find it hard to believe that anybody here will be angry that cops are taking their time and doing a good job.

"With these girls, with these victims, please, take your time and get justice for these women and these families," she said, addressing authorities. "We don't care how long this has to be here."

Cops have been digging in Heuernmann's yard by hand while also removing bags of items from the property and dismantling a wooden deck.

Police have searched two nearby storage units for human remains or other possible clues in the chilling case, too.

Heuermann, a married father of two, was nabbed outside his Midtown Manhattan office July 13 and charged in the deaths of Melissa Barthelemy, Megan Waterman and Amber Lynn Costello, whose bodies were found along a marshy stretch of land in 2010.

Cops believe he is also linked to the death of Maureen Brainard-Barnes — the last of the "Gilgo Four."

Seven other bodies were also discovered near the site, which remain part of unsolved cases.

All four women tied to Heuermann worked as escorts.

Authorities have said the suspect was a frequent solicitor of sex.

In the days after Heuermann's arrest, police removed from his family home a truckload of items, including a creepy child-sized doll with blond braides encased in glass and wood, a portrait of a disfigured woman and a cache of guns — as many as 300, according to cops, photos and reports.

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