How can I get Cook-Cook's head?

In Fallout: New Vegas, on the Three Card Bounty side quest, I have the other two heads, however, I don't have Cook-Cook's head. When I go back to check his body, I can't get his head. I did kill the three BEFORE I activated the quest, so I'm not sure if that ruined it for me or not. He has been decapitated, so I can see his head rolling around, but I can't pick it up. Is this a glitch, or have I just screwed myself over?