Sonny Pierce was charged back in April of 2011 but never went to trial.
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He was already in jail on charges of sexually assaulting a 15-year-old girl in 2010.
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He has now pleaded guilty in that case and the murder of 18-year-old Kiara Windom of Harvey.
Prosecutors say Pierce sexually assaulted Windom, strangled her and dumped her body in an alley in 2009.
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Pierce was also charged in the murders of 18-year-old Kimika Coleman, of Chicago, and 17-year-old Mariah Edwards, of Blue Island, however, charges in the deaths of the two other teens were dropped.
In 2011, prosecutors said Pierce confessed on tape and investigators found video of Pierce having sex with one of his teeange victims. Prosecutors said Pierce met the women on dating sites and lured them to his apartment, then killed them.
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Pierce will be sentenced in October.