Inside this Oklahoma prison a death row inmate died of a heart attack during a botched execution.<br/> <br />Clayton Lockett, who was only 38 years-old, experienced a vein failure which prevented the lethal injection of three drugs from being fully effective.<br/> <br />The execution was halted when it became obvious something had gone wrong and penetentiary workers blocked the view of the guerney.<br/> <br />(SOUNDBITE) (English) MEDIA WITNESSES, SAYING:<br/> <br />"At 6:39 he's still lifting his shoulders and head, off the gurney, grimacing. And appeared to be in distress."<br/> <br />(SOUNDBITE) (English) MEDIA WITNESSES, SAYING:<br/> <br />"So from 6:23, the execution started. 6:39 is when they closed the curtains on us. A 16-minute process to watch."<br/> <br />The failed execution is likely to raise more questions about the lethal injection and whether it is humane.<br/> <br />Lockett was sentenced to death here for shooting and eventually killing a woman by burying her alive.<br/> <br />The execution of fellow inmate Charl
