Texas executed a man convicted of killing a police informant on Tuesday after the US Supreme Court rejected claims that he was too mentally impaired to face the death penalty.
Marvin Wilson's lawyers pointed to a 2004 test which gave him an IQ of 61, below the US's usual "minimum competency standard" of 70.
Texan attorneys said the test finding was not consistent with "other assessments."
Defence lawyer Lee Kovarsky said it was "outrageous" that the state utilised "unscientific guidelines" to determine who is exempt from execution.
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