Huntsville, Texas - Texas carried out its 600th execution since 1982 on Thursday, after the Supreme Court rejected the condemned person's argument that intellectual disability made him ineligible for the death penalty.

Edward Lee Busby Jr. was pronounced dead following a lethal injection for the 2004 death of 77-year-old retired university professor Laura Lee Crane.

In his last statement, Busby apologized to his family and Crane's family and asked for forgiveness, according to a transcript provided by the Texas Department of Criminal Justice.

"Ms. Crane was a lovely woman, I never meant anything bad to happen to her. I am so sorry," he was quoted as saying.

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Busby's lawyers had tried to halt the execution, citing findings that Busby was intellectually disabled. Executing such an individual would violate a constitutional ban on "cruel and unusual" punishment.

"Whether Busby is intellectually disabled is a question that the Supreme Court has repeatedly made clear should be grounded in the opinions and findings of experts and not judicially crafted findings and opinions. Unfortunately, to this point, the experts have been ignored," the clemency petition read.

The conservative-dominated US Supreme Court on Thursday overturned a lower court's stay of execution.

It was the 12th execution in the US this year.

Busby became the 600th person in Texas executed since 1982, when the state resumed capital punishment following a nationwide moratorium.

Texas is the busiest death penalty state in the country, executing more than four times as many people as another other state, according to the Death Penalty Information Center.

The death penalty has been abolished in 23 of the 50 states, while three others – California, Oregon, and Pennsylvania – have moratoriums in place.