An Arizona judge has ruled that a death row inmate convicted of killing a university student in 1978 failed to prove he was mentally incompetent to understand his fate, setting the stage for the state's first execution since 2014.

Pinal County Superior Court Judge Robert Olson wrote that Clarence Wayne Dixon, convicted of murdering Deana Bowdoin, suffered from schizophrenia but had shown "sophistication, coherent and organized thinking" despite claiming he believed he was being executed for a 1985 sexual assault crime.

The United States Supreme Court has ruled that execution of mentally disabled people is unconstitutional.

Convicted of homicide, Dixon was sentenced to death in 2008 in the fatal stabbing and strangulation of Bowdoin, a student at Arizona State University.

The case was unsolved until 2001, when investigators matched Dixon's DNA with evidence found at the scene. At the time, Dixon was serving life in prison for a 1986 sexual assault, a different incident than the 1985 crime.

Dixon is scheduled to die by lethal injection on May 11 in Arizona's first execution since Joseph Wood in 2014.

Wood's lawyers said the state botched that execution after it took him two hours to die from a two-drug cocktail injection. Arizona halted executions before a federal judge in 2017 accepted sweeping reforms of its death penalty protocols.

The state agreed to stop using certain drugs for executions. It also said it would limit the authority of the director of the department of corrections to change drugs and allow a prisoner time to challenge any drug changes.

Several state governments and the U.S. federal government have struggled in recent years to obtain lethal-injection drugs, while legal and ethical questions swirl around capital punishment.