Campaigners have asked Missouri's governor to halt the execution of a mentally impaired man who was convicted of raping and pushing two sisters to their death in 1991.
The execution of Antonio Richardson has also been opposed by the mother of the victims. He is set to die on Wednesday at the Potosi Correctional Centre.
The case has drawn international attention because Richardson was 16 at the time of the crime, and is borderline mentally retarded with an IQ of 70.
Groups such as Amnesty International, the Children and Family Justice Centre, the Association for Retarded Citizens, Human Rights Watch and the European Union have asked Governor Bob Holden to spare Richardson's life.
Ginny Kerry, the mother of the victims, said: "I requested clemency for him because of his youth and his diminished mental capacity".
Her former husband and other family members have said they want the execution to proceed.
Kerry's daughters, Robin, 19, and Julie, 21, were on an abandoned Mississippi River bridge with a cousin on April 4, 1991, when they encountered four young men. The men attacked and robbed them, raping the girls before pushing them off the bridge. Their 19-year-old cousin, Tom Cummins, was ordered to jump and survived.
Along with Richardson, Marlin Gray, then 23, and Reginald Clemons, who was 19 at the time of the crime, were sentenced to death for the killings. The fourth man, Daniel Winfrey, then 15, pleaded guilty to second-degree murder in exchange for his testimony.