Arizona carried out its first execution since 2014 on Wednesday, putting to death a prisoner convicted of killing a student more than four decades ago, state prison officials said.

Clarence Wayne Dixon, convicted of fatally stabbing and strangling Arizona State University student Deana Bowdoin in 1978, was put to death by lethal injection at 10:30 a.m. local time at the state prison in Florence, Arizona, Frank Strada, deputy director of the state's department of corrections, told a news conference.

In his last words Dixon maintained his innocence and criticized a U.S. Supreme Court ruling allowing his execution to proceed, Strada said.

The U.S. Supreme Court on Wednesday denied without explanation a request from Dixon's lawyers to halt the execution on grounds of insanity.

It was the first time Arizona carried out a death sentence since the 2014 execution of Joseph Wood when it took nearly two hours for a two-drug cocktail injection to take effect, and witnesses said he snorted and gasped before he died. Corrections authorities said Wood was comatose and never in pain.

Afterwards, Arizona halted executions. In 2017, a federal judge accepted sweeping reforms of the state's death penalty protocols, including an agreement to stop using certain drugs. The state 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 drugs used in lethal injection drugs, while legal and ethical questions swirl around capital punishment.

Dixon was sentenced to death in 2008, some 30 years after Bowdoin was killed. The case went 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.

Pinal County Superior Court Judge Robert Olson ruled last week that Dixon was mentally fit to be executed. The U.S. Supreme Court has ruled that execution of mentally disabled people is unconstitutional.