BOSTON July 11, 2013 -- A water bottle recovered from a construction site where Tim DeSalvo – whose uncle Albert DeSalvo had confessed to being the internationally notorious Boston Strangler – gave police the DNA evidence they needed to bring closure to a case that has been a mystery for nearly 50 years, murders for which no one has ever been charged.

"This is really a story of relentlessness,'' Boston Police Commissioner Ed Davis explained today as Massachusetts top law enforcement officials revealed that DNA preserved from the body of the Boston Strangler's last victim--raped and murdered in 1964--can now be linked with "99.9 percent certainty" to the late Albert DeSalvo.

"This is good evidence. This is strong evidence. This is reliable evidence,'' Suffolk County District Attorney Dan Conley said of the new DNA result. "But there can be no doubt."

The Boston Strangler case, which inspired a 1968 Hollywood movie starring Tony Curtis, marked a terrifying swath of history in the city – and one that has long been mired in doubt.

It was the 1960s and single women across Massachusetts were the target of a serial killer and rapist. When it was over, the Boston Strangler had killed 11 women. The case baffled the five separate District Attorney's offices investigating the murders because of the spread-out locations of the victims. Then DeSalvo, a convicted rapist, made a jailhouse confession claiming that he was the Boston Strangler and provided details on the 11 murdered women.

But DeSalvo was never charged in the case and was found dead in his cell under mysterious circumstances at Walpole state prison in 1973. This week investigators will exhume DeSalvo's body from Puritan Lawn Cemetery in Peabody, Mass., to make the final determination with DNA testing.

The strangler's last victim would be 19-year-old Mary Sullivan, strangled with her own stocking in her Beacon Hill apartment on tony Charles Street. She was also sexually savaged.

Her killer left behind seminal fluids that were lifted from a maroon blanket her body was covered with. That remains the only DNA evidence in the entire Boston Strangler investigation: six samples that the Boston Police Crime Lab's lead forensic scientist Robert Hayes preserved as he waited for technology to advance to the point where nuclear DNA could be positively matched to a suspect.

First, police had to make sure the Y-chromosomes in those DNA samples were a familial match to DeSalvo in order to convince a judge to let investigators disturb his grave. So BPD Sgt. Brian Albert, a surveillance expert, followed nephew Tim DeSalvo to his worksite in Boston and retrieved a water bottle he drank from and left behind. It was a match to the samples collected in the 1964 Beacon Hill murder, excluding 99.9 percent of the male population from suspicion in Mary Sullivan's killing, Hayes said, and pointing to Albert DeSalvo with near certainty as the man responsible.

"I knew science would one day provide us with answers in this case,'' Hayes told ABC News.

Those answers provided comfort to the nephew Mary Sullivan never met: Boston author Casey Sherman, who had long held that his aunt had been murdered not by DeSalvo but by another man. He even wrote a book, "A Rose for Mary" about the investigation he launched to assuage his mother's nightmares. His mother Diane was just 17 when Mary Sullivan was murdered and she continued to dream of her sister, Sherman told ABC News.

"I am grateful this brings closure to me and to my mother most of all,'' Sherman said, his voice shaking with emotion. He got choked up, took a breath, and continued talking.

"For all these years it was just me and her chasing this case,'' Sherman said. "It took 49 years for police to say they legitimately got him."

But Elaine Sharpe, a lawyer for the DeSalvo family, insisted that police have not legitimately identified Albert as the Boston Strangler. She added that his nephew did not know he had been followed and inadvertently provided the evidence for the search warrant that will lead to the body being exhumed 30 years after it was buried.

"Just because they had DNA,'' Sharpe said, "Doesn't mean Albert DeSalvo killed her."

Massachusetts Attorney General Martha Coakley dismissed that assertion, saying: "We may have solved one of the nation's most notorious serial killings."