The Manhattan District Attorney’s Office is looking into the mysterious death 51 years ago of newspaper writer and “What’s My Line?” star Dorothy Kilgallen, who was investigating the JFK assassination, The Post has learned.

The stunning development comes after a new book, “The Reporter Who Knew Too Much,” suggests Kilgallen was murdered to shut down her relentless pursuit of a Mafia don linked to JFK and Lee Harvey Oswald.

Joan Vollero, a spokeswoman for DA Cyrus Vance Jr., confirmed that a staffer has read the book, and reviewed a letter from author Mark Shaw citing new leads, medical evidence, and witnesses overlooked when Kilgallen, 52, died suddenly on Nov. 8, 1965, at the peak of her career.

“I’m hopeful DA investigators will probe any records available and interview witnesses still alive today who can shed light on what happened to this remarkable woman,” Shaw told The Post, which featured his findings last month.

“Victims have rights whether their name is Dorothy Kilgallen or Dorothy Doe, and Kilgallen was denied justice in 1965. That’s why I’m fighting for her.”

Shaw said he has received dozens of emails from readers demanding an official investigation. One “called her ‘a patriot’ who should be revered for risking her life to solve the JFK assassination.”

Kilgallen, who wrote a widely syndicated column for the New York Journal-American, was the only reporter ever to interview Jack Ruby, who shot Oswald, and published Ruby’s closed-door testimony to the Warren Commission before its official release. Her enemies ranged from Frank Sinatra to FBI Director J. Edgar Hoover.

The morning after Kilgallen appeared on the hit TV game show, she was found dead in her Manhattan apartment, naked under a robe and still in make-up. The medical examiner ruled it an accidental mix of booze and sleeping pills.

But Shaw contends Kilgallen was drugged. He cites a powdery residue on a glass by the bed, and records obtained from the National Archives showing two additional barbiturates in her system.

“There was no evidence that Kilgallen was a drug abuser,” Shaw said Saturday. “Despite the odd death scene and heavy doses, there was no investigation.”

Former ME toxicologist Dr. Stephen Goldner told Shaw the Mafia controlled the Brooklyn ME’s Office, which inexplicably conducted the Kilgallen autopsy even though her death occurred in Manhattan.

Weeks before her death, Shaw learned, Kilgallen bought a gun for self-protection and planned a second trip to New Orleans to investigate Mafia don Carlos Marcello.

“If the wrong people knew what I know about the JFK assassination, it would cost me my life,” she confided to hairdresser Charles Simpson, one of several witnesses who gave videotaped interviews unearthed by Shaw.

Shaw urges the DA to interview Ron Pataky, an Ohio newspaper columnist and ex-lover seen huddling with Kilgallen at the Regency Hotel the night before she died. Pataky, now 81, later penned a poem called “Vodka Roulette,” typed next to the image of a bartender mixing drinks, that reads, “Make one of ’em poison.”