Medical opinions notwithstanding, Warren Hill is scheduled to die by lethal injection on Tuesday. Georgia Dept. of Corrections

Warren Hill has been diagnosed with mental retardation by 10 different physicians. But on Tuesday, the Georgia death row inmate is scheduled to face execution by lethal injection, even though, according to the U.S. Supreme Court, such an act technically qualifies as cruel and unusual punishment.

In a final effort to stay his impending execution, lawyers for Hill, 53, are using new evidence to emphasize their client is severely mentally disabled and, therefore, cannot constitutionally be given the death penalty. Three doctors who originally testified for the state of Georgia, at the time alleging Hill was competent to face capital punishment, have come forward in the last year to say their initial evaluation was wrong.

“It was out of the blue; I didn’t expect the experts to suddenly change their mind,” Brian Kammer, Hill’s lead attorney, said in a phone interview.

After the Georgia State Board of Pardons and Parole refused to commute Hill’s sentence to life without parole in July 2012, one of the three psychiatrists who originally said the prisoner did not meet the criteria for mental retardation contacted Kammer to say he was concerned about his previous diagnosis.

“He felt like it was a rushed process and wanted to revisit the case,” Kammer said. “At the time of the trial back in 2000, the doctor had never even diagnosed mental retardation in a patient before.”

Hill, who was found to have an IQ of 70, was initially diagnosed with borderline intellectual functioning -- instead of the more severe mental retardation -- by those physicians, even though seven others determined he was actually mentally disabled.

According to the affidavit testimony of Dr. Thomas Sachy, the first to contact the defense, at the time of the 2000 evidentiary, he was still a relatively inexperienced psychiatrist who was not as familiar with the “science pertaining to the issues in Mr. Hill’s case.”

Hill’s defense team hopes to use the new information to prove the inmate is, beyond a reasonable doubt, afflicted with mental retardation. While Georgia eliminated capital punishment for the mentally disabled in 1988, it is the only state in the union that requires retardation to be proved “beyond a reasonable doubt.”

The closest the defense team has come to reaching that threshold was in 2002, when a state judge agreed there was a preponderance of evidence indicating Hill has mental retardation. However, the Georgia Supreme Court overturned that finding in 2003, and a federal appeals court ruled against Hill in 2011.

Although federal law prohibits the execution of the mentally disabled, the nation’s high court left it to the states to determine a prisoner’s sanity.

Hill has been on death row for more than 20 years for the 1990 killing of his prison cell mate, Joseph Handspike. Authorities say Hill, who was already serving a life sentence for the 1986 slaying of his girlfriend, used a board studded with nails to bludgeon Handspike to death.

Handspike's family has publicly declared they do not believe Hill should be put to death.

Although Hill was originally set to be executed in July, the Georgia Supreme Court granted him a stay while his lawyers pursed a challenge based on the state’s changing of execution method directly before the defendant’s scheduled date. But in early February, the court found the decision to replace a three-drug combination with one drug for executions is not subject to the Georgia Administrative Act, meaning that changing the execution method does not require public hearings, as Hill’s lawyers had argued.

At least five men with recorded histories of mental incompetence have faced execution in the last year, according to the Death Penalty Information Center.

Hill is scheduled to die Tuesday at 7 p.m. His lawyers have already asked the U.S. Supreme Court to halt the execution.