Investigators have been spotted digging up the backyard of Rex Heuermann’s home amid reports that the Gilgo Beach serial killer suspect may have killed at least one victim inside a sound-proofed room in his basement. Search teams carried out what was described as a “major excavation” on Sunday at the one-storey home in Massapequa Park, Long Island, where the 59-year-old architect lived with his wife and two adult children. Cadaver dogs and a ground-penetrating radar machine were brought in to scour the ground before excavators were seen digging up the earth with a backhoe. At one point, a huge wooden deck was dismantled and a blue tent was erected to shield the search from view. A neighbour told The New York Post on Sunday that authorities had found a sound-proofed room in the basement of the home and that Mr Heuermann is believed to have killed at least one of his victims down there. “He’s got a soundproof room in his basement,” Robert Musto, a 64-year-old longtime neighbour said he had been told by law enforcement on 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 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 former coworker confirmed the existence of a soundproof basement room to the paper, claiming that the accused serial killer once took time off work in order to carry out the bizarre construction of the vault in his family home. The individual, who was not named, recalled Mr Heuermann building the concrete-lined vault which is around two to three feet thick. “it’s not just a hidden room – it’s a serious vault,” they 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.” New York officials have not confirmed the existence of the sound-proofed basement room or publicly said that they believe Mr Heuermann may have killed one or more victims in the room. However, officials are probing the possibility that the accused serial killer killed the victims inside his family home. His wife and two children were out of town at the time of each of the three murders he is charged with. Law enforcement sources previously told CNN that a vault of some sort had been located in the basement of the Massapequa Park home – and that a trove of up to 300 firearms had been found inside. The married father-of-two had 92 legal gun permits and was a keen hunter, according to police. In the 11 days since Mr Heuermann was arrested in connection to the infamous serial killer case, investigators have been searching for evidence tying him to the murders including any trophies taken from the victims. As well as extensively searching his home, officials are probing other locations connected to the accused murderer including storage units two miles away in Amityville. Meanwhile, law enforcement agencies are also looking into unsolved murders and missing persons cases all across America including in Las Vegas, South Carolina and Atlantic City. Over in South Carolina, Chester County Sheriff’s Office said that they were looking for evidence to assist New York officials. “The Chester County Sheriff’s Office was requested by the Gilgo Beach Task Force to assist in gathering evidence in Chester County relevant to their investigation,” the department tweeted. Mr Heuermann owns a property in Chester next to his brother Craig. The pickup at the centre of the murder investigation was seized from his brother’s home last week. Mr Heuermann was arrested on the night of 13 July when a team of officers swooped as he left his office in Midtown Manhattan where he ran an architecture business. He was charged with three counts of murder in the first degree and three in the second degree over the deaths of Megan Waterman, Melissa Barthelemy and Amber Costello. He is also the prime suspect in the murder of Maureen Brainard-Barnes – who together with the three is known as the “Gilgo Beach Four” and was last seen alive in early June 2007 in New York City. The four women were found within one-quarter mile of each other, bound by belts or tape and some wrapped in burlap – their bodies dumped along Gilgo Beach. He has pleaded not guilty to the charges and is being held without bond. Court records show that Mr Heuermann was linked to the killings through a pimp’s tip about his pickup truck, a stash of burner phones, “sadistic” online searches, phone calls taunting victims’ families, his wife’s hair found on the victims’ bodies – and a pizza crust. The first piece of the puzzle came when a witness in the Amber Costello case revealed details about a vehicle that a client was driving when she was last seen alive. Costello, who worked as a sex worker, was seen alive on the evening of 2 September 2010 when she left her home in West Babylon. A witness said she had gone to meet a client who was driving a first-generation Chevrolet Avalanche. Last year, a registration search showed that local man Mr Heuermann owned a first-generation model of the truck at the time of Costello’s disappearance. He also matched the witness’ description of the man believed to be the killer: a large, white “ogre”-like male in his mid-40s, around 6’4’ to 6’6” tall, with “dark bushy hair,” and “big oval style 1970’s type eyeglasses”. The discovery of the car led investigators to hone in on Mr Heuermann including executing 300 subpoenas, search warrants and other legal processes to obtain evidence to determine his potential involvement in the killings. Among this was Mr Heuermann’s alleged use of burner phones, with prosecutors saying that he used burner phones to contact the three women and arrange to meet them at the time when they went missing. He also allegedly took two of the victims’ cellphones – and used one to make taunting phone calls to one of their families where he boasted about her murder, court documents state. Mr Heuermann’s DNA was found on one of the victims, while his wife’s hairs were found on three of the four women he is connected to. His arrest comes after the horrific serial killer case has captured the nation’s attention for more than a decade. The Gilgo Beach murders had long stumped law enforcement officials in Suffolk County who believed it could be the work of one or more serial killers who targeted sex workers and dumped their bodies along the remote beaches on Ocean Parkway. The case began in May 2010 when Shannan Gilbert vanished after leaving a client’s house on foot near Gilgo Beach. She called 911 for help saying she feared for her life and was never seen alive again. During a search for Gilbert in dense thicket close to the beach, police discovered the remains of another woman. Within a matter of days, the remains of three more victims were found close by. By spring 2011, the remains of a total of 10 victims had been found including eight women, a man, and a toddler. Police have long thought that it could be the work of one or more serial killers. Gilbert’s body was then found in December 2011. Her cause of death is widely contested with authorities long claiming that it is not connected to the serial killer or killers but that she died from accidental drowning as she fled from the client’s home. However, an independent autopsy commissioned by her family ruled that she died by strangulation and her mother believes she was murdered. Like Gilbert, most of the victims targeted were sex workers while some are yet to be identified. Read More More families await answers in Gilgo Beach killings – and the names of other victims Pizza crust, burner phones and his wife’s hair: How Long Island police tied Rex Heuermann to the Gilgo Beach murders BTK killer makes chilling comparisons between himself and Gilgo Beach murders suspect Rex Heuermann