PROVINCETOWN, Mass. — Federal agents working with genetic genealogists have solved the 48-year-old mystery of an unidentified woman found beaten and mutilated on a Massachusetts beach in 1974.

“The Lady of the Dunes” has been identified as Ruth Marie Terry, 37, of Tennessee, authorities announced on Monday. According to the FBI, Terry’s was the oldest case of an unidentified homicide victim in Massachusetts.

“This is, without a doubt, a major break in the investigation that will, hopefully, bring all of us closer to identifying her killer,” said Joseph Bonavolonta, special agent in charge of the FBI’s Boston field office. “Earlier this morning, FBI special agents and victim specialists, along with troopers from the Massachusetts State Police delivered the news to the victim’s family.”

Bonavolonta asked that the family be given privacy to grieve.

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The long-unsolved case began on July 26, 1974, when a 13-year-old girl walking her dog along Race Point Beach at the Cape Cod National Seashore stumbled upon a gruesome sight.

Among the dunes, in a patch of scrub pine a mile east of the Race Point ranger station, was the naked, decomposing body of a woman. She was lying face down on a beach blanket, with her Wrangler jeans folded neatly and placed under her head like a pillow.

The woman, who had been dead for at least a week, had a large hole in her left temple where her skull had been crushed by a blunt object, FBI officials said. The…