The US law forbids the execution of prisoners judged to be mentally incompetent.
The convicted murderer, Robert Holsey, was not given a sufficient opportunity to prove his disability because standards set by that state for the determination of mental competency are unreasonably strict, "which creates an unacceptable risk of wrongful execution of the intellectually disabled," the inmate's attorney said Tuesday as quoted by USA Today.
"Clearly, a less onerous standard of [mental deficiency] proof would have meant the difference between life and death for Mr. Holsey," the inmate's attorney said Tuesday as quoted by USA Today.
Holsey was judged competent to stand trial because he had an IQ of 79, and "understood complicated legal concepts, and had a sophisticated vocabulary," the news outlet quoted a state judge as saying.
Sentenced to death in 1997, Holsey, 49 at the time of his execution, killed a deputy sheriff in 1995 following the robbery of a convenience store. The Supreme Court denied a request for a stay of execution and the prisoner was executed Tuesday night.