The relevant passages are:
[Ennis] "There was two old guys ranched together down home, Earl and Rich ... I wasw, hwat nine years old and they found Earl dead in an irrigation ditch. They took a tire iron to him, spurred him up, drug him around ..."
[Jack] "You seen that?"
[Ennis] "Dad made sure I seen it. ... Hell, for all I know, he done the job."
--
This would be all right, Jack would answer, had to answer. But it he did not. It was Lurleen and she said who? who is this? and when he told her again she said in a level voice yes, Jack was pumping up a flat on the truck out on a back road when the tire blew up. The bead was damaged somehow and the force of the explosion slammed the rim into his face, broke his nose and jaw and knocked him unconscious on his back. By the time someone came along he had drowned in his own blood.
No, he thought, they got him with the tire iron.
(and when he visits her): No doubt about it, she was polite but the little voice was cold as snow.
--
And the last page:
Around that time Jack began to appear in his dreams ... but the can of beans with the spoon handle jutting out and balanced on the log was there as well ... The spoon handle was the kind that could be used as a tire iron. And he would wake up sometimes in grief, sometimes with the old sense of joy and release...
--
So as far as Ennis is concerned, from whose point of view the story is told, it was the tire iron.
Hope that helps.
~Alyssa
