US killer dies of heart attack after FORTY MINUTES writhing in agony and sitting up to say 'something's wrong' when experimental drug execution causes his vein to explode

                                               The execution of Clayton Lockett, pictured right, was halted about 20 minutes after the first of three experimental drugs was administered. He received the entire cocktail of injections - which had never been tried in Oklahoma before - but for some reason the drugs didn't kill him. At one point, Lockett sat up and said 'something's wrong' but the prison officials only lowered the blinds in the execution chamber. The inmate finally suffered a massive heart attack and died at 7.06p.m. A four-time felon, Lockett, 38, was convicted of shooting 19-year-old Stephanie Neiman, shown left, with a sawed-off shotgun and watching as two accomplices buried her alive in rural Kay County in 1999 after Neiman and a friend arrived at a home the men were robbing.