The so-called “Lady of the Dunes” murder mystery has finally been solved after nearly 50 years — with prosecutors identifying her now-dead husband as the killer.

Ruth Marie Terry, 37, was only formally identified last fall as the woman whose bludgeoned and almost decapitated body was found in sand dunes in Provincetown in July 1974.

That breakthrough finally allowed investigators to eye Terry’s husband, Guy Rockwell Muldavin, as the killer, Cape and Islands District Attorney Robert Galibois told NBC10 on Monday.

“Based on the investigation into the death of Ms. Terry, it has been determined that Mr. Muldavin was responsible for Ms. Terry’s death in 1974. Mr. Muldavin passed away in 2002,” Galibois said in a statement.

The case is now officially closed, Galibois added.

Muldavin was the last person to see Terry alive — and had also been a suspect in the 1960 murders of another wife and her daughter, but was never charged with their homicides, NBC10 said.

Terry’s remains were already in an advanced stage of decomposition when they were found by a 12-year-old girl, the Cape Cod Times noted.

Even so, police were able to determine that the victim had been killed by blunt force trauma and nearly decapitated, and had her hands severed, the outlet explained.

There was no sign of a struggle on or near the body, indicating that the deceased likely knew her attacker.

However, Terry remained unidentified for over 40 years — instead known only;y as the “Lady of the Dunes.”

She was finally identified in Oct. 2022 after her jaw was tested using genetic genealogy at the Othram forensics lab, the Cape Cod Times previously reported.

Terry’s identity led investigators to Whitwell, Tennessee, where she was born, and to Reno, Nevada, where records show she married Muldavin in Feb. 1974, NBC10 said.

Muldavin – who also went by Raoul Guy Rockwell and Guy Muldavin Rockwell – had earlier been a suspect in the April 1960 killing of his second wife, Manzanita Mearns Rockwell, and her daughter, Dolores, after their bodies were found in a septic tank.

He remarried just months after the murders, the reports said.

He was never charged with their murders, but served 15 months behind bars after being charged with unlawful flight for not giving testimony about the mutilation of human remains.

It is unclear when Terry and Muldavin met, but she brought him to meet her family in Whitwell in April 1973, according to NBC10.

Terry visited home alone in April 1974, just a few months before her body was found.

A few months later, in late summer 1974, Muldavin returned to Tennessee to tell Terry’s family that she was missing.

By that time, however, her body had already been discovered in the dunes.

Two years after Terry’s murder, Muldavin also published a disturbing book, “Cooking with Rump Oil,” that included a sick recipe with troubling similarities to the crime, NBC10 revealed.

The recipe, “Cape Cod, Shid,” features an illustration of a long-haired creature alongside instructions including,  “After the Shid is caught anything over five minutes ends it! The sweet turpentine taste will turn to that of a burnt glove and the tender look will become one of despair.”

Retired FBI profiler Julia Cowley called the bizarre passage “really horrifying.”

“What I do wonder — especially the last line, ‘the tender look will become one of despair’ — you have to think that perhaps was the moment he watched the life go out of her eyes and when she realized, ‘He’s going to kill me,’” Cowley told NBC10.

Muldavin died in California in 2002 at age 78, the outlet said.

Terry’s biological son, RIchard Hanchett, previously said he only learned his birth mother’s name in 2018, when a DNA test led him to his maternal relatives in Whitwell.

At the time, Terry’s relatives believed she was missing.

“A horrible thing happened to my mom in a beautiful place. She was a beautiful person and I wish I could have gotten to know her,” he previously told NBC10.

As of Dec. 2022, the family planned to bury Terry alongside her mother, Eva, who died when she was a child, the outlet said.

The Cape and Islands District Attorney did not immediately return The Post’s request for comment Tuesday.