Patrick Jon Zacharias pleaded guilty on Tuesday to raping and murdering Kay Lynn Jackson in Boise in 1998.
Jackson, 22, was killed near the Greenbelt under the Americana Boulevard bridge while walking to church.
Boise police detectives investigated the case for 14 years until finally getting a break when DNA evidence led to Zacharias who has been in prison since 2007, serving a life sentence for lewd conduct with a girl under 11.
In April 2013, a grand jury indicted Zacharias for Jackson's murder. A judge in June entered a plea of not guilty on Zacharias' behalf as the defendant chose to "stand silent" rather than enter a plea.
Zacharias has agreed to plead guilty if the state agrees not to seek the death penalty.
Under the plea agreement, Zacharias will serve a life sentence without parole after he serves his current life sentence. He also will waive all appeals.
Sentencing is set for Jan. 17 before Judge Ronald WIlper, who must approve the plea agreement.