US Supreme Court rejected arguments that Marvin Wilson was too mentally challenged to qualify for death penalty.

A Texas man convicted of killing a police informant has been executed after the US Supreme Court rejected arguments that he was too mentally impaired to qualify for the death penalty.

Marvin Wilson, 54, was pronounced dead late on Tuesday, 14 minutes after his lethal injection began at the state prison in Huntsville.

He was convicted of fatally shooting a police drug informant nearly 20 years ago in Beaumont.

Wilson’s lawyers had argued that he should have been ineligible for capital punishment because of his low IQ.

They cited a US Supreme Court ruling that banned capital punishment for the mentally impaired.

In their appeal to the high court, the lawyera pointed to a psychological test conducted in 2004 that pegged Wilson’s IQ at 61, below the generally accepted minimum competency standard of 70.

But lower courts agreed with state attorneys, who argued that Wilson’s claim was based on a single test that may have been faulty, and that his mental impairment claim was not supported by other tests and assessments of him over the years.

Stay of execution

The Supreme Court denied his request for a stay of execution less than two hours before his lethal injection began.

Lee Kovarsky, lead defence lawyer, said he was “gravely disappointed and saddened” by the ruling, calling it “outrageous that the state of Texas continues to utilise unscientific guidelines … to determine which citizens with intellectual disability are exempt from execution”.

Commenting on the case, Simon Whitaker, a clinical psychologist from the UK, told Al Jazeera earlier on Tuesday: “This is a miscarriage of justice, he [Wilson] would have the same reasoning powers of a five-year-old child, if a five-year-old child killed someone we would not execute them in part.

“Texas courts have judged Wilson as not mentally retarded – even though psychologists measured his IQ in the bottom one percentile, about equal to that of an average five-year-old child.

“With his diagnosis he cannot make rational decisions, there will be a clear problem for him to think things through,” said Whitaker.

Shot in the head

Wilson was convicted of murdering 21-year-old Jerry Williams in November 1992, several days after police seized 24 grammes of cocaine from Wilson’s apartment and arrested him.

Witnesses testified that Wilson and another man, Andrew Lewis, beat Williams outside of a convenience store in Beaumont, about 128km east of Houston.

Wilson, who was free on bond, accused Williams of snitching on him about the drugs, they said.

Witnesses said Wilson and Lewis then abducted Williams, and neighbourhood residents said they heard a gunshot a short time later.

The next day, Williams was found dead on the side of a road, wearing only socks, severely beaten and shot in the head and neck at close range.

‘Skirting the ban’

The Supreme Court issued a ruling in 2002 outlawing the execution of the mentally impaired, but left it to states to determine what constitutes mental impairment.

Kovarsky argued that Texas is trying to skirt the ban by altering the generally accepted definitions of mental impairment to the point where gaining relief for an inmate is “virtually unobtainable”.

State attorneys say the court left it to states to develop appropriate standards for enforcing the ban and that Texas chose to incorporate a number of factors besides an inmate’s IQ, including the inmate’s adaptive behaviour and functioning.

Edward Marshall, a Texas assistant attorney general, said records show Wilson habitually gave less than full effort and “was manipulative and deceitful when it suited his interest”, and that the state considered his ability to show personal independence and social responsibility in making its determinations.

Laura Moye, director of Amnesty International’s US death penalty abolition campaign, told Al Jazeera that her group was “disturbed and disappointed” by the failure of the Supreme Court to intervene in Wilson’s case.

“We’re not saying he shouldn’t have been held responsible for his part in a terrible crime, but that it’s an egregious act in a country that prides itself on its human rights record to put to death a man such as this one,” Moye said hours after Wilson was executed.

“There are important human rights standards that the United States ought to adhere to, particularly because it’s enshrined in our constitution that there are certain people who shouldn’t face execution.”

Texas is one of 33 states, along with the federal government, which still maintains the death penalty. A recent poll indicated more than 70 per cent of Texans approve of capital punishment for murder.