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Albert DeSalvo says he murdered 13 single women in the Boston area between June 1962 and January 1964, creating a climate of fear in the city.
The women, aged between 19 and 85, were sexually assaulted and then strangled to death in their homes.
Some were found with trademark ribbons around their necks.
But the 35-year-old has not been charged with any of the murders because of a lack of evidence.
During the seven-day trial, DeSalvo's lawyer, F. Lee Bailey, attempted to prove that his client was guilty of the murders, and should be found insane and sent to a psychiatric hospital for life.
Mr Bailey described DeSalvo as "uncontrollable" and sending him to prison would be a cruel punishment.
He said: "There were 13 acts of homicide by a completely homicidal vegetable walking in the form of a human being."
But the jury found DeSalvo legally sane and not guilty of the murders.
The judge said: "This defendant must be incarcerated for as long as he shall live or until psychiatric science can cure him."
'What I had done'
The former military police officer has been held on charges of rape in the Bridgewater State Hospital in Massachusetts since 1964.
He will be kept there pending an appeal against his conviction.
Mr Bailey has said that shortly after DeSalvo arrived at the hospital, which has a designated section for patients with criminal records, he told other inmates that he murdered the women.
DeSalvo has said: "I would go home and watch what I had done on TV. Then I would cry like a baby."
Because DeSalvo's police record was filed under "breaking and entering", he never came under suspicion during the murder hunt.
Detectives, pathologists and psychologists were investigating known sex offenders.
Some women in Boston were so terrified by the murders that they carried pepper, ammonia and tear-gas bombs to protect themselves.
Shortly after he was sentenced, DeSalvo caused panic when he briefly escaped from the hospital in Massachusetts where he was being held.
He was then moved to the maximum security Walpole State Prison where he was stabbed to death in November 1973.
Prosecutors claimed DeSalvo died because he was trying to enter the prison's drug trade.
Three inmates were tried for the killing but none was convicted.
Casey Sherman, the nephew of the Boston Strangler's last victim, 19-year-old Mary Sullivan, believes DeSalvo confessed to the murders because it would lead to a lucrative book and film deal.
He says DeSalvo was killed after he told his psychiatrist he was going to tell the truth.
Mr Sherman and DeSalvo's family say there is no physical evidence to link him to the crimes and he did not match any witness descriptions of possible suspects.
Also, when the murders happened the police did not believe one individual was responsible.
But Julian Soshnick, a prosecutor who worked on the investigation, has said that during interviews DeSalvo described in detail facts about the killings that were not known to the public.
These include what ligatures were used and how they were tied around the victims' necks.
In December 2001, DeSalvo's body was exhumed and tests showed his DNA did not match evidence found on the body of Miss Sullivan.
Mr Soshnick says this does not prove DeSalvo is innocent of the 19-year-old's murder or any of the other killings.
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