Marvin Wilson, 54, was executed by lethal injection on Tuesday evening local time in the state prison in Huntsville, Texas.

Wilson had been sentenced to death in 1994 for killing a police informant in 1992.

His lawyers claimed he had an IQ of 61, which disqualified him from execution under a US Supreme Court ruling of 2002. However, the ruling gave states some leeway in deciding who qualified for protection.

The state of Texas had argued that a 2004 IQ test on Wilson had come up with a result that was too low, citing several other tests showing an IQ above 70, the level sometimes used as a marker for mental retardation. A last-minute application to the Supreme Court for a stay of execution was denied.

Human rights group Amnesty International called the decision to go ahead with the execution "highly disturbing," and several other rights organizations have also criticized the sentence.

Wilson's lawyer, Lee Kovarsky, said earlier on Tuesday that it was "outrageous that the state of Texas continues to utilize unscientific guidelines... to determine which citizens with intellectual disability are exempt from execution."

Another African American diagnosed as having a mental disorder, Yokamon Hearn, 34, was executed in Texas last month despite international protests.

Wilson is the 25th person to be executed in the US this year and the seventh in Texas, which has carried out more than a third of all executions in the country since 1982.

The death penalty is currently a legal sentence in 33 of the 50 US states.

tj/slk (AFP, dpa, Reuters)