HOUSTON - A mentally retarded armed robber was executed Tuesday night by lethal injection in the US state of Texas, contradictory to a 2002 Supreme Court ruling.

Marvin Wilson, 54, was pronounced dead shortly after execution, reported The Houston Chronicle on its website. He was found guilty in 1992 of abducting and killing police informant Jerry Williams.

In 2002, the Supreme Court ruled that mentally retarded criminals should be barred from execution as it is unconstitutional. Since Wilson scored 61 in an IQ test, nine points below the standard for competency, his lawyers had tried to save him from execution, citing the Supreme Court ruling.

Wilson's attorney Lee Kovarsky voiced dissatisfaction over the execution of the killer, saying that "It is outrageous that the state of Texas continues to utilize unscientific guidelines to determine which citizens with intellectual disability are exempt from execution."