The pounding on the door of the unkempt Massapequa Park house late on the night of July 13, 2023, changed the lives of Rex Heuermann’s family forever.

FBI agents and local cops forced their way in and upended the lives of the accused Long Island serial killer’s wife and two grown children, just hours after the hulking architect was arrested outside his Manhattan office in the grisly cold-case murder of three women.

Their beloved father is a monster.

“And I just hear banging: ‘FBI, open up!'” Victoria Heuermann, the accused killer’s daughter, said in her first public interviews in the new Peacock docu-series, “The Gilgo Beach Killer: House of Secrets.” “This isn’t happening. This isn’t happening.

“I knew my dad had stuff with the IRS, Dept. of Labor. But the FBI?” she said. “And they said we have evidence your dad murdered all these women and your home is now a crime scene. I never would imagine I’d hear that in my life.”

Rex Heuermann, 61, now stands charged with the murder and mutilation of seven sex workers who disappeared more than 30 years and were dumped in desolate stretches of Long Island.

Suffolk County prosecutors maintain that the accused killer kept a detailed “how-to” document on his computer to plan the gruesome slayings — and may have slaughtered the women in the family basement while his wife and daughters were away.

They suggested the murders could have happened inside a concrete gun vault in the family’s basement, where Heuermann, a gun enthusiast, stored as many as 300 weapons.

To his wife and children, news of his alleged exploits shattered their lives in one night.

Asa Ellerup, Rex Heuermann’s wife, recalled waiting up for her husband to return from his Midtown office on the day of the FBI raid, when the knock on the door came around 11 p.m.

“I was waiting for my husband, but he didn’t come home. And then suddenly I see them come on the front porch,” she said. “They push their way in and they asked me, do you know why we’re here. And I said, my husband’s guns?

“And the woman investigator says to me, ‘Have you heard about the murders on Gilgo Beach?’ And I said, ‘I heard of them, but I don’t know anything. And she said, ‘Your husband, he knows all about it,'” Ellerup recalled. “They’re telling me that my husband is alleged to be some horrific serial killer

“And then the investigator says to me my husband will be charged in the morning with murdering three women,” she said in the documentary. “And I told her she’s crazy.”

Ellerup, Victoria and her half-brother Christopher Sheridan were all kicked out of the house and into a nearby hotel with little more than the clothes on their backs — but only after being grilled by the feds.

“They started asking about if my dad was ever sexually advancive towards mem or touched me, and I said no, I cannot really recall any time he was like that,” Victoria Heuermann said.

“For the first time in my life, I’m not waking up in the morning and going to work. I actually don’t even know what tomorrow was even gonna look like.”

What followed was the first of two intrusive and thorough searches of the house that saw local, state and federal authorities rummage inside and dig out the backyard in a hunt for evidence.

Suffolk County District Attorney Ray Tierney said DNA evidence from hairs found on the bodies of all seven victims linked back to the Heuermanns — including a sample found on a pizza box outside the Manhattan office and a sample snatched from a soft drink Victoria tossed into a trash can.

Heuermann, 61, stands accused of killing seven sex workers since 1993 and dumping them along Ocean Parkway — with the remains found between 2010 and 2011.

The murders remained unsolved until his 2023 arrest in the deaths of Amber Lynn Costello, Megan Waterman and Melissa Barthelemy. In January 2024 he was also charged with killing Maureen Brainard-Barnes, with the four women collectively known as the “Gilgo Four” among locals.

Tierney ultimately charged Heuermann with three other murders — Valerie Mack, Jessica Taylor and Sandra Costilla, the first victim, who was killed in 1993.

Heuermann pleaded not guilty to all of the charges lodged against him.

His lawyer, Michael Brown, has asked the judge to separate at least some of the murder cases to have them tried separately, and has raised doubts about the DNA evidence.