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

Cops reportedly found a soundproof room in the basement of accused Gilgo Beach serial killer Rex Heuermann’s home where they believe at least one woman may have been killed – and brought in cadaver dogs, a backhoe and ground-penetrating radar to try to locate possible bodies and “trophies” in his backyard.

New York State Troopers and Suffolk County Police have been combing the home, yard and storage sites of the accused killer’s family for 11 days since the hulking architect was charged in the deaths of three women and publicly named a prime suspect in the fourth – all part of the infamous 13-year-old “Gilgo Four” murders.

“This guy is crazy,” Robert Musto, 64, a Long Island Rail Road retiree and longtime neighbor of Massapequa Park, told The Post on Sunday, referring to the 59-year-old suspect.

“He has a soundproof room in his basement,” Musto told officers at the scene. “What do you think it was for?

“They say there’s evidence he killed at least one of the girls there,” the Long Island neighbor said. “The cops are going to dig all this up. They said they were concentrating on the soundproof room in the basement but were going to look into everything.

A law enforcement source told the Post on Sunday, “Cops are looking for evidence if the victims were in the house, nothing at this time.”

An investigator was seen moving meticulously across the ground in the backyard on Sunday with a radar device resembling a lawnmower, video footage showed.

Three cadaver dogs were also brought to the property, along with a backhoe and a dump truck for what state troopers at the scene called a “major excavation.”

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

“We don’t care how long it has to be here,” Huber, 57, said on Sunday. It’s a big community of cops and firefighters, and I find it hard to believe anyone here will be mad that the cops take their time and do a good job.

“With these girls, with these victims, please take your time and get justice for these women and families,” she said, addressing the authorities. “We don’t care how long it has to be here.”

Cops dug by hand into Heuernmann’s yard while removing bags of items from the property and dismantling a wooden deck.

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

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

Cops believe it 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 among the unsolved cases.

The four women linked to Heuermann worked as escorts.

Authorities said the suspect was a frequent sex advocate.

In the days following Heuermann’s arrest, police removed a truckload of items from his family home, including a creepy child-sized doll with blonde braids encased in glass and wood, a portrait of a disfigured woman and a cache of guns – up to 300, according to cops, photos and reports.

Police also seized a Chevrolet Avalanche from the driveway and a second Avalanche from a remote property he owns in South Carolina, the same model that ultimately helped cops catch him.

Heuermann’s wife, who police say is “disgusted” and “embarrassed” by his arrest, filed for divorce last week and remains in hiding despite being spotted a few times with her children and outside her lawyer’s office.

Heuermann pleaded not guilty to the charges in court, while his attorney has since suggested police ignored other “more significant” clues leading to other suspects.

“It’s pretty crazy to hear,” said Tom Donelson, a 51-year-old Nassau County court clerk who lives in Mineola and was watching the scene of the case on Sunday.

Musto and other community residents described the “ogre”-looking Heuermann as quiet — and right next to it.

“Detectives, when they questioned me, they told me that everyone here told him the same thing as me: a tight-knit community and he doesn’t interact with anyone,” Musto said.

Additional reporting by Larry Celona