Texan man Robert Ladd, 57, was executed on Thursday evening by lethal injection in the state's death chamber in the city of Huntsville, just hours after the US Supreme Court rejected claims that he was mentally impaired and thus ineligible for the death penalty.

Ladd was sentenced to death in 1997 for the rape and murder of a 38-year-old woman, carried out in 1996 while he was on parole for a separate triple murder in 1978.

His lawyers had launched an appeal at the Supreme Court citing a psychiatrist's assessment from 1970 that Ladd, then 13 and in custody of the Texas Youth Commission, had an IQ of 67. Many US courts have accepted scientific studies citing an IQ of 70 as the upper threshold of impairment.

Ladd was also in a specialist center for mentally handicapped people by the age of 18, according to a court document.

'Well-documented deficits'

"Ladd's deficits are well documented, debilitating and significant," Brian Stull of the American Civil Liberties Union's capital punishment project said.

"Anywhere else in the country, Mr Ladd's IQ of 67 would have meant a life sentence, not death," he added.

The claim of Ladd's impairment was dismissed by Kelli Weaver, a Texas attorney general, who told the justices in a filing that "each court that has reviewed Ladd's claim has determined that Ladd is not intellectually disabled."

Another of Ladd's attorneys, Maurie Levin, had also tried to gain a stay of execution by questioning whether the poison used for executions in Texas, pentobarbital, was strong enough to ensure that the person executed does not suffer.

That appeal was also rejected by the Supreme Court, which last year turned down a review of Ladd's case.

Texas top executor

Ladd's execution is the second this week in the United States. On Tuesday, Warren Hill, a double killer whose lawyers also claimed he was intellectually disabled, was executed in the state of Georgia, after the Supreme Court also rejected last-minute appeals for clemency.

Texas is the US state with the highest rate of executions in the country and has put to death 520 convicts since the death penalty was reinstated in 1976.

tj/gb (AFP, AP)