In 2007, Prieto's defense lawyers appealed to the US Supreme Court, claiming that the convict's intelligence quotient was below the state standard for mental retardation, exempting him from capital punishment. The jury rejected Prieto's intellectual disability claim.
He had been on death row since 1992 for the rape of a minor in 1990 in California, and in 2010, Prieto was convicted of two capital murders and a rape in 1988 in Virginia. He has reportedly been connected to six more murders in the two states.
More than 800 people have been executed in the United States in the past 15 years, according to the US Death Penalty Information Center non-profit organization. The largest number of executions, 85, occurred in 2000. Thirty-five people were executed in the United States in 2014.
Capital punishment in general, and especially the practice of convicting persons with mental health issues, has drawn much criticism from both local and international human rights groups.