Accident. All references to Jack's death are either from the narrator "Ennis didn't find out about the accident until months later" or from Ennis's point of view, which is referred to in a hallucinatory way. Ennis had absolutely no way to "know for certain" that it was a tire iron. However, he was certain that living openly with another man would lead to his and/or Jack's certain death and the comment by Jack's father about Jack planning to leave his wife and move in with "this other fella" fit his paranoid conviction. It was a measure of Ennis's paranoia that he so wilfully believed in Jack's murder.
I agree with this interpretation, for just those reasons. Also, there was foreshadowing in the story that Ennis would respond that way. When he and Jack were in the motel, he said that if his dad saw them "you bet he'd go get his tire iron."
Some people say, what are the chances of Jack being killed in such a freak tire accident? Well, slim. But that kind of explosion does happen. Are they any smaller than the chances that Jack would be killed
the exact same way that Ennis had always feared?
Plus, wouldn't Jack's body have been examined by someone who could tell the difference from the effects of an accident as Lureen describes and the effects of what Ennis imagines?
Ennis has a good reason to suspect murder -- he'd been fearing it all his life. But what is OUR reason, as readers, for suspecting it? All we know is that Ennis suspects it. If Ennis had said, "Oh, OK," in response to Lureen's account and never considered the idea of murder, we'd have no reason to suspect murder, either. But again, we know Ennis is paranoid, so why are his fears a good basis for ours?
The story works better for me as literature if Jack's death is accidental. It's much more ironic and sad to think that Ennis' paranoia is so strong that it leaves him eternally uncertain.
I used to be in the "we aren't meant to know" club, but more recently I've come around to this view.