A law firm hired by Kathie Durst’s family has accused the NYPD of colluding with an ex-cop to cover up Robert Durst’s role in her death — leaving him free to kill at least two other people.

The real estate scion’s first wife went missing in 1982 and was officially declared dead last year. A lawsuit filed Thursday claims cops are hiding evidence that shows they intentionally protected her abusive hubby from prosecution.

The Manhattan Supreme Court filing says Durst’s late father, real-estate mogul Seymour Durst, paid ex-cop Edward Wright to conduct a “shadow investigation” that kept his family abreast of the NYPD’s probe.

Wright’s efforts allegedly revealed that cops “knew or should have known” Durst had repeatedly lied about “his involvement in Kathie’s disappearance and murder.”

But in February 1983, NYPD Lt. Robert Gibbons publicly proclaimed that Durst “is not a suspect — nor has ever been — in his wife’s disappearance.”

At the time, Gibbons — described in the suit as “one of Durst’s biggest advocates” — also said that cops were still “actively investigating the case.

Court papers say there’s “not even a scintilla of proof” to back up that claim, and blast Gibbons for his “inexplicable exoneration” of Durst.

The blockbuster allegations are contained in a suit that seeks to force the NYPD to cough up all its records regarding Kathie’s still unsolved disappearance on Jan. 31, 1982.

The plaintiff in the suit is lawyer Bob Abrams’ firm — Abrams Fensterman — which is representing Kathie’s kin in a related, $100 million suit that accuses Durst of interfering with their right to give her a prompt burial.

The new suit says the NYPD has been stonewalling a request for its records under Freedom of Information Law for nearly 18 months.

“The NYPD knows that once its cover-up is fully and properly exposed, the public will likely conclude that the NYPD intentionally concealed evidence to shield Durst from being prosecuted in connection with the disappearance and/or death of Kathie,” court papers say.

The suit also says the department has blood on its hands for Durst’s admitted, self-defense killing of a Texas rooming-house neighbor and his alleged, execution-style slaying of a potential witness against him in his wife’s case.

Those “violent and horrific crimes…would likely never have occurred had the NYPD conducted an honest, competent and timely investigation in connection with Kathie’s disappearance and murder,” court papers say.

Durst — who’s awaiting trial in Los Angeles in the Christmas Eve 2000 assassination of college pal Susan Berman — has denied any involvement in Kathie’s disappearance.

But on March 16, 2015, he was infamously heard muttering “What the hell did I do? Kill them all, of course,” during the climax of the six-part HBO documentary “The Jinx.”

Durst was arrested in New Orleans hours before the final episode aired, and told authorities he was high on methamphetamine when he was interviewed for the mini-series, according to court records made public last year.

“I was on meth, I was on meth the whole time … it should have been obvious,” Durst said.

He also said of his hot-mic confession: “I think the reason I did it had to be because I was swooped, speeding.”

In addition to Durst’s “killed them all” admission, Thursday’s suit cites his remarks in “The Jinx” regarding Wright, who retired from the NYPD in 1973 and later worked as chief investigator for the state Attorney General’s Organized Crime Task Force.

During the series, Durst bragged that the late Nicholas Scoppetta — a high-powered lawyer who served as fire commissioner under then-Mayor Michael Bloomberg — had hired Wright on his behalf.

“Ed Wright was able to get lots of stuff from the police. He knew the police. The police liked him. He was able to get information that was theoretically unavailable. Who thought what? Who said what,” the suit quotes Durst saying.

In the suit, Abrams argues that while Durst “had real time access to the investigation of him by the police, the NYPD cannot now claim that they are unable to provide… access to the NYPD’s file when the NYPD has already given it to, at a minimum, people working on behalf of Durst.”

Neither the NYPD, Wright nor Gibbons immediately returned requests for comment.