Attorneys for a death row inmate say his pending execution is unconstitutional because he never fatally shot a suburban Dallas police officer during a Christmas Eve robbery more than 18 years ago.

Patrick Murphy's lawyers say the 57-year-old shouldn't be executed Thursday night because he wasn't a major participant in the December 2000 robbery in which Irving police Officer Aubrey Hawkins was killed by the notorious "Texas 7" gang of escaped prisoners.

If Murphy's appeals fail and he is executed, he'd be the fourth person put to death in the U.S. this year.

Murphy was convicted under Texas' law of parties, which holds a person criminally responsible for the actions of another if they are engaged in a conspiracy.