A prisoner who has been behind bars for decades for a St. Louis-area killing has pleaded guilty to one of four other deaths he has been linked to by prosecutors.

Gary Muehlberg, 73, pleaded guilty to first-degree murder and was sentenced to life in prison Monday in the 1990 death of 21-year-old Sandy Little, the St. Louis Post-Dispatch reported.

Prosecutors said in September that Muehlberg had confessed to killing Little and three other women in 1990. The confessions came after prosecutors promised not to seek the death penalty.

He has served 28 years of a life sentence for the 1993 killing of Kenneth Atchison, of St. Louis County. Prosecutors said he killed Atchison for $6,000 and put his body in a plywood box in Muehlberg's Bel-Ridge basement.

Little's body was found in an abandoned box off Interstate 70 in O'Fallon five months after she disappeared in September 1990.

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Muehlberg has plea hearings scheduled this month in the deaths of Robyn Mihan, Brenda Pruitt and Donna Reitmeyer.

The four women disappeared from south St. Louis within six months in 1990. Their bodies were found by passersby in makeshift containers in Lincoln, St. Charles and St. Louis counties: One was bound between two mattresses, one was concealed in a wooden box and two were stuffed into garbage cans.

Prosecutors said all of the women's killings were connected to an area of south St. Louis that was known at the time for prostitution but authorities have not released a motive for their deaths.

The cases were unsolved until O'Fallon police Detective Sgt. Jodi Weber reopened the investigations in 2008. She eventually found DNA on one of the bodies that matched Muehlberg's.