Police secretly followed DeSalvo's nephew to collect DNA from a discarded water bottle to help make the connection, officials said. Conley said the match excludes 99.9 percent of suspects, and he expects investigators to find an exact match when the evidence is compared directly with DeSalvo's DNA.
The district attorney stressed that the evidence only applied to Sullivan's slaying and not the other 10 homicides.
"Even among experts and law enforcement officials, there is disagreement to this day about whether they were in fact committed by the same person," Conley said.
Sullivan, 19, had moved from her Cape Cod home to Boston just days before her death. She was found strangled in her Boston apartment in January 1964 and has long been considered the strangler's last victim.
Eleven Boston-area women between the ages of 19 and 85 were sexually assaulted and killed between 1962 and 1964, crimes that terrorized the region and grabbed national headlines.
Thursday's announcement represented the first forensic evidence tying DeSalvo to the case.
DeSalvo, a blue-collar worker and Army veteran who was married with children, confessed to the 11 Boston Strangler slayings, as well as two others. But he was never convicted of the Boston Strangler killings.
He had been sentenced to life in prison for a series of armed robberies and sexual assaults and was stabbed to death in the state's maximum security prison in Walpole in 1973 — but not before he recanted his confession.
An attorney for DeSalvo's family said Thursday they believe there's still reasonable doubt he killed Sullivan, even if additional DNA tests show a 100 percent match.
The lawyer, Elaine Sharp, said previous private forensic testing of Sullivan's remains showed other DNA from what appeared to be semen was present that didn't match DeSalvo.
"Somebody else was there, we say," Sharp said of the killing. "I don't think the evidence is a hundred percent solid, as is being represented here today."
But Donald Hayes, a forensic scientist who heads the Boston Police Department's crime lab, said investigators' samples were properly preserved, while the evidence used in private testing came from Sullivan's exhumed body and was "very questionable."
Sharp also said Thursday that the family was outraged that police followed a DeSalvo relative to get the DNA they needed for comparison.
Casey Sherman, a nephew of Sullivan's who wrote a book on the case pointing to other possible suspects, acknowledged the new findings point to the man he had defended. Sherman said the DNA evidence against DeSalvo appeared to be overwhelming.
"I only go where the evidence leads," he said, thanking police and praising them "for their incredible persistence."
Sherman also expressed sympathy for the DeSalvo family, with whom he had aligned in the past in a shared belief that DeSalvo didn't kill his aunt. That belief was based on DeSalvo's confession, which Sherman previously said was inconsistent with other evidence.
The families of DeSalvo and Sullivan had jointly sued the state for release of evidence while pursuing their own investigation. Sullivan's body was exhumed in 1999 for private DNA testing as part of the effort.
F. Lee Bailey, the attorney who helped to obtain the confession from DeSalvo, said Thursday's announcement will probably help put to rest speculation over the Boston Strangler's identity.
Bailey had been representing another inmate who informed the attorney that DeSalvo knew details of the crimes. Bailey went to police with the information, and he said DeSalvo, who was already in prison for other crimes, demonstrated he knew details only the killer would know.
Bailey would later represent DeSalvo.
"It was a very challenging case," said Bailey, who lives in Yarmouth, Maine. "My thought was if we can get through the legal thicket and get this guy examined by a team of the best specialists in the country, we might learn something about serial killers so we could spot them before others get killed."
Authorities said they're continuing to comb through evidence files and still are hoping to find samples to do DNA testing in connection with the other Strangler-linked killings.
They plan to exhume DeSalvo's body from a grave in Peabody, Mass., sometime this week and said it could be just a matter of days before they get results of DNA testing they're planning.
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