FLORENCE, Ariz. (AP) — An Arizona man convicted of killing a college student in 1978 was put to death Wednesday after a nearly eight-year hiatus in the state’s use of the death penalty brought on by an execution that critics say was botched as well as the difficulty state officials faced in finding lethal injection drugs.

Clarence Dixon, 66, died by lethal injection at the state prison in Florence for his murder conviction in the killing of 21-year-old Arizona State University student Deana Bowdoin, making him the sixth person to be executed in the U.S. in 2022. Dixon’s death was announced late Wednesday morning by Frank Strada, a deputy director with the Arizona Department of Corrections, Rehabilitation and Reentry.

Dixon’s death appeared to go smoothly, said Troy Hayden, an anchor for the Fox10 TV news program who witnessed the execution.

“Once the drugs started flowing, he went to sleep almost immediately,” Hayden said.

In the final weeks of his life, Dixon’s lawyers made last-minute arguments to the courts to postpone his execution, but judges rejected his argument that he isn’t mentally fit to be executed and didn’t have a rational understanding of why the state wanted to execute him. The U.S. Supreme Court rejected a last-minute delay of Dixon’s execution less than an hour before his execution began.

Dixon had declined the option of being killed in the gas chamber — a method that hasn’t been used in the U.S. in more than two decades — after Arizona refurbished its gas chamber in late 2020. Instead, he was executed with an injection of pentobarbital.

Strada said Dixon’s last statement was: “The Arizona Supreme Court should follow the laws. They denied my appeals and petitions to change the outcome of this trial. I do and will always proclaim innocence. Now, let’s do this (expletive).”

The last time Arizona executed a prisoner was in July 2014, when Joseph Wood was given 15 doses of a two-drug combination over two hours in an execution that his lawyers said was botched. Wood snorted repeatedly and gasped more than 600 times before he died.

States including Arizona have struggled to buy execution drugs in recent years after U.S. and European pharmaceutical companies began blocking the use of their products in lethal injections.

Authorities have said Bowdoin, who was found dead in her apartment in the Phoenix suburb of Tempe, had been raped, stabbed and strangled with a belt.

Dixon, who was an ASU student at the time and lived across the street from Bowdoin, had been charged with raping Bowdoin, but the rape charge was later dropped on statute-of-limitation grounds. He was convicted of murder in her killing.

In arguing that Dixon was mentally unfit, his lawyers said he erroneously believed he would be executed because police at Northern Arizona University wrongfully arrested him in another case — a 1985 attack on a 21-year-old student. His attorneys conceded he was lawfully arrested by Flagstaff police.

Dixon was sentenced to life in prison in that case for sexual assault and other convictions. DNA samples taken while he was in prison later linked him to Bowdoin’s killing, which had been unsolved.

Prosecutors said there was nothing about Dixon’s beliefs that prevented him from understanding the reason for the execution and pointed to court filings that Dixon himself made over the years.

Defense lawyers said Dixon was repeatedly diagnosed with paranoid schizophrenia, regularly experienced hallucinations over the past 30 years and was found “not guilty by reason of insanity” in a 1977 assault case in which the verdict was delivered by then-Maricopa County Superior Court Judge Sandra Day O’Connor, nearly four years before her appointment to the U.S. Supreme Court. Bowdoin was killed two days after that verdict, according to court records.

Another Arizona death-row prisoner, Frank Atwood, is scheduled to be executed on June 8 in the killing of 8-year-old Vicki Lynne Hoskinson in 1984. Authorities have said Atwood kidnapped the girl.

The child’s remains were discovered in the desert northwest of Tucson nearly seven months after her disappearance. Experts could not determine the cause of death from the bones that were found, according to court records.

Arizona has 112 prisoners on death row.