Convicted Milwaukee serial killer Walter Ellis dies in South Dakota

SOUTH DAKOTA -- A Milwaukee serial killer convicted of murdering seven women over 21 years died Sunday.

Ellis, 53, died of apparent natural causes at a Sioux Falls hospital. He was housed at the South Dakota State Penitentiary under an interstate compact agreement, where he was serving consecutive life sentences for first-degree murder, according to the South Dakota Department of Corrections.

Ellis was arrested in 2009 after his DNA matched semen samples found on six homicide victims. He raped and strangled seven women between 1986 and 2007.

The case brought major attention to the way DNA samples are collected by the state from convicted felons. Ellis' DNA was missing from a state database because he was able to convince another inmate to provide a DNA sample on his behalf in 2001.

Ellis pleaded no contest and was found guilty of seven counts of first-degree intentional homicide in February 2011.

An autopsy will be done, according to officials.