Attorneys for a twice-convicted murderer are asking the U.S. Supreme Court to stop his upcoming execution in Texas, saying he's mentally impaired.

Robert Ladd is scheduled for execution Thursday. The 57-year-old was convicted of strangling and beating 38-year-old Vickie Ann Garner before setting her body on fire in her Tyler apartment in 1996.

Ladd had pleaded guilty in 1980 to a similar slaying in Dallas, where a woman's body was burned in a blaze that also killed her two children. Although he received a 40-year prison term, he was paroled after serving only about a third of his sentence.

Ladd's attorneys say juvenile records show that a psychiatrist determined a 13-year-old White had an IQ of only 67 and that he can't fully function on his own.