U.N. expert says U.S. executions, due Wednesday, unconstitutional

GENEVA (Reuters) - A United Nations human rights investigator called on the U.S. states of Georgia and Texas to halt the execution of two men, scheduled for Wednesday, saying that the prisoners were mentally disabled and killing them would breach the U.S. constitution.

Christof Heyns, U.N. special rapporteur on extrajudicial, summary or arbitrary executions, said that it was a "violation of death penalty safeguards" to execute someone suffering from "psychosocial disabilities" and said their death sentences should be commuted.

"It is also contrary to the U.S. Supreme Court ruling Atkins v. Virginia which held that such executions are unconstitutional," Heyns said in a statement, referring to a 2002 ruling that bans executing mentally retarded defendants.

Warren Hill and Yokamon Laneal Hearn were found guilty of murder in Georgia and Texas, respectively, and their convictions have been the subject of a number of legal appeals based on the defendants' mental health, he said.

The Georgia State Board of Pardons and Paroles rejected clemency for Hill on Friday.

Heyns voiced particular concern that "Georgia is now the only state in the United States that requires proof of what it calls 'mental retardation beyond a reasonable doubt', rather than a preponderance of the evidence as in other jurisdictions".

"This higher standard of proof, making it very difficult to demonstrate that one actually suffers from a psychosocial disability, may, I fear, mean that Mr. Hill, scheduled for execution tomorrow (Wednesday), would be a fatality in violation of international as well as domestic law," he said.

Heyns said there was evidence to suggest that Hearn, who faces execution in Texas, suffers from psychosocial disabilities, "including an expert opinion that he is affected by structural brain dysfunction likely to have been caused by his mother's alcohol abuse during pregnancy".

Amnesty International said in a report in March that the United States carried out 43 executions in 2011, down from 46 a year earlier. The group cited the case of Eddie Powell, executed in Alabama in June 2011 "despite evidence that he had a degree of 'mental retardation' which would render his execution unconstitutional".

The United States was the only country in the Americas and the only member of the Group of Eight leading economies to execute prisoners last year, according to Amnesty.

(Reporting by Stephanie Nebehay; Editing by Robin Pomeroy)