This subject has been floating around for a while now, vaguely hinted at in some older threads, and nearly implied in others.
Perhaps the most convincing argument I have seen in the story is the passage where John Twist is telling Ennis that Jack planned to divorce Lureen and move back with the Texas ranch neighbor. Immediately after this passage, the story proceeds to say that at that point Ennis then knew it was the tire iron that killed Jack.
Some have taken this as a reference to Ennis assuming that Jack was murdered because of the impending divorce with Lureen. It is indeed strange that Proulx would have Ennis come to this realization (i.e., that Jack was murdered) as a consequence of John Twist's revelation of Jack's intended plans to move back.
I do not have a copy of the online version of the story, but someone with access to one can reference this passage in question.
This also lends support to some interpretations of Lureen's bitter last words to Ennis over the phone: 'I suppose they [i.e., Jack's parents] would be pleased if his wishes were carried out, about the ashes, I mean.' (I paraphrase from memory.)
Some have interpreted this as a double edged sword. Lureen having to clarify that 'his wishes' pertained to 'the ashes' implies that 'his wishes' could be interpreted in another way. The implication of course, is that Lureen, having realized at that point who Ennis really was, mockingly asked him to get in touch with Jack's folks, so that the ashes can finally come ot the possession of the one person that Jack wished to be united with in death: Ennis.
I do not have a particularly strong opinion for or against this interpretation. But I would be interested in your thoughts...