To confirm the match, investigators unearthed his remains a week ago and said Friday that the odds that the semen belonged to a male other than DeSalvo were 1 in 220 billion.
"It's a great day. This is now full justice for my aunt, Mary Sullivan," said her nephew, Casey Sherman.
A lawyer for DeSalvo's family, Elaine Sharp, said last week that even a perfect DNA match wouldn't mean he killed Sullivan and suggested that someone else was present at the slaying. She said previous private testing on Sullivan's remains showed the presence of DNA from what appeared to be semen that wasn't a match to DeSalvo.
Police responded last week by saying the evidence used in private testing from Sullivan's exhumed remains was "very questionable."
Sharp also said in a statement that DeSalvo's brother and his nephew whom police secretly trailed to collect a family DNA sample from a discarded water bottle won't comment on the new DNA result because it hasn't been proven to be relevant to the question of whether DeSalvo raped and strangled Sullivan.
"There is no level of 'unprecedented certainty' as now claimed by the government," Sharp said.
But the idea that the DNA match doesn't identify DeSalvo as Sullivan's killer is bizarre, responded Jake Wark, a spokesman for Suffolk District Attorney Daniel Conley.
"It suggests that Mary Sullivan had consensual sex with Albert DeSalvo moments before another person who has never been identified sexually assaulted and strangled her to death, leaving no trace of his presence," Wark said. "Frankly, it defies everything we know about this case."
Sullivan was 19 when she died in January 1964.
Law enforcement officials disagree about whether the same person killed all the women whose deaths became connected to the Strangler. DeSalvo went to prison for life for a series of armed robberies and sex assaults before someone fatally stabbed him in 1973.
F. Lee Bailey, a defense lawyer who once represented DeSalvo, said Friday that DeSalvo provided so many details that only the perpetrator would know that he became convinced that his client was the Boston Strangler.
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Associated Press writer David Sharp in Portland, Maine, contributed.