The ‘Lady of the Dunes,’ whose remains were discovered on Cape Cod nearly half a century ago, was killed by her husband, authorities announced Monday, bringing one of Massachusetts’ most notorious and longstanding cold cases to an official close. Ruth Marie Terry, 37, was only identified as the victim last October, after DNA analysis and genealogy research lead to a break in the case. On Monday, Cape and Islands District Attorney Robert Galibois confirmed that Guy Rockwell Muldavin, long the prime suspect in the case, “was responsible” for Terry’s death. Muldavin and Terry had been married for just five months before her naked body was discovered in 1974, her hands severed in an attempt to evade fingerprint identification, officials said. Her skull had been crushed and she had nearly been decapitated. “She was a very beautiful woman, we all loved her,” Terry’s sister-in-law told The Boston Globe on Monday. “I just never dreamed it would turn out this way.” Muldavin, also suspected of murdering another of his wives and a stepdaughter years earlier, died in 2002.