Ellis, 53, died apparently of natural causes, prison officials said. A former detective said he took the answers to other suspected killings with him.
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Walter Ellis eluded Milwaukee investigators for decades while women were strangled on the city's north side. On December 1, 2013, less than three years into his life sentence, he died.
Ellis, 53, died at a Sioux Falls, South Dakota, hospital, apparently of natural causes, the South Dakota Department of Corrections said, and an autopsy was planned. He had been hospitalized since November 26, according to corrections spokesman Michael Winder. Ellis was serving his sentence in a South Dakota prison under an agreement with Wisconsin. Steven Spingola, a former Milwaukee homicide detective who worked on two of the killings, said there were "at least five, maybe seven more" cases in which Ellis was a good suspect. "Why he wanted to kill these women it'll go with him to his grave," Spingola said.
Ellis pleaded no contest to first-degree intentional homicide in the deaths of seven women, ages 16 to 41, killed from 1986 to 2007, and in February 2011 he was sentenced to seven consecutive life terms without parole. Among the victims was Joyce Mims, 41, killed in 1997. After his 2009 arrest, police said his DNA matched samples from six of the victims and blood on a can of pepper spray found where the seventh was killed. His DNA had been missing from the state database because, authorities said, he persuaded a fellow inmate to submit a sample in his place. Two other men, Chaunte Ott and William Avery, were convicted in two of the killings before they were freed.
When Ellis was arrested in 2009, police said his DNA had been found on the bodies of nine women, but he was convicted of killing seven.