Authorities have reportedly discovered a soundproof room in the basement of Rex Heuermann’s alleged Gilgo Beach serial killer home where they suspect at least one woman may have been killed. They have also reportedly used ground-penetrating radar, cadaver dogs to search for possible bodies and ‘trophies’ in his backyard.

Since the hulking architect was charged in the deaths of three women and publicly named a prime suspect in the fourth, all of which were part of the infamous 13-year-old ‘Gilgo Four’ slayings, New York State troopers and Suffolk County police have been searching the accused killer’s family home, yard and storage sites.

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“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 the 59-year-old suspect.

“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.”

The Post was informed on Sunday by a law-enforcement source that “Cops are looking for evidence if the victims were in the house, nothing yet.”

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A radar tool that looked like a lawn mower was seen being carefully manoeuvred around the ground by an investigator in the backyard on Sunday, according to video evidence.

In addition, three cadaver dogs, a digger, and a dump truck were brought to the area for what the state troopers on the scene described as a “major excavation.”

The neighbourhood seems ok with the interruption, according to local Kathy Huber, who claimed to have attended high school with Heuermann.

Cops have been manually excavating in Heuernmann’s yard, hauling away bags of stuff and tearing down a wooden deck.

Two adjoining storage lockers have also been checked by police for any potential human remains or other clues in this terrifying case.

The bodies of Melissa Barthelemy, Megan Waterman, and Amber Lynn Costello were discovered along a marshy piece of property in 2010. Heuermann, a married father of two, was apprehended outside his Midtown Manhattan office on July 13 and charged in their deaths.