A man convicted of slaying two people and critically injuring a third in a drug house shooting was executed on Tuesday evening by Texas officials, despite evidence that he suffered from mental retardation.

Milton Mathis, 32, was sentenced to death in 1999, three years before the U.S. Supreme Court ruled that execution of the mentally retarded violated the Constitution's prohibition on cruel and unusual punishment.

Intelligence tests, including one given by the Texas Department of Corrections in 2000, measured Mathis's IQ in the low 60s, well below the threshold for mild mental retardation as recognized by almost all states.

In 2005, however, a Texas court rejected his claims of mental impairment, siding with prosecutors who characterized Mathis as a "street smart" criminal whose behavior indicated near-normal intelligence. Federal and state courts declined to overturn the verdict, clearing the way for his execution by lethal injection at the Texas Department of Criminal Justice in Hunstville. A last-ditch petition by Mathis's attorneys requesting a stay of execution and a review of his case was rejected without comment by the Supreme Court late Tuesday afternoon.

Mathis was pronounced dead at 6:53 p.m.

"The system has failed me," he said in a final statement, according to prison officials.

A spokeswoman for Texas Governor Rick Perry, a Republican who is weighing a run for the presidency, said the governor could not offer clemency or a reprieve in the case without a positive recommendation from the Texas Board of Pardons and Paroles, which voted on Monday to reject a reprieve for Mr. Mathis. Members of the board are appointed by the governor.

In 2001, Gov. Perry vetoed a bill passed by the Texas legislature banning the execution of the mentally retarded, saying that the state's judicial system already contained adequate protections for such defendants. Supporters of the bill disagreed, pointing to evidence indicating that at least a half-dozen prisoners with mental deficiencies had been executed since 1990.

Since taking office in December 2000, Gov. Perry has overseen more than 230 executions, more than any other U.S governor in modern history. Mathis was the 6th inmate put to death in Texas this year, and the 23rd in the nation.

In an editorial last week in the Dallas Morning News, former Texas governor Mark White (D) called on Perry to authorize a temporary reprieve for Mathis to examine his claims of mental retardation.

"Mathis has suffered from obvious mental disabilities since childhood," wrote White. "He failed the first, fifth and eight grades and dropped out of high school in ninth grade."

"The governor of Texas is authorized by law to take action to prevent precisely this sort of injustice," he wrote.

Fred Felcman, an assistant district attorney for Fort Bend County who led the prosecution of Mathis, disputed White's claims, saying Mathis's mental deficits were not severe enough to disqualify him from the death penalty.

"We don't execute people who are mentally retarded," Felcman said. "The guy is street smart."

Felcman attended the execution in Huntsville at the request of Melanie Almaguer, he said, who was paralyzed from the chest down at age 15 after being shot in the face by Mathis.

Steven Rocket Rosen, who defended Mathis in his original trial, said there was "no excuse" for the actions of his client. But he said Mathis's mental problems were severe and had been aggravated by heavy drug use from a young age.

According to court records, Mathis began smoking PCP and marijuana soaked in formaldehyde, known as "fry," as early as age 12.

"The guy is off, way off," Rosen said. "He's retarded. They should have offered life in this case and got on with it."