Heya,
I'm posting my answer here before I read other replies as I cast my vote here. So, I apologize if I reiterate something that's been said here already.
My main answer is that we're not supposed to know. Of course neither the film nor the story give us enough evidence to definitively say one way or the other. (And, after this, for the record what I say here is based on the film.. since I'm more of a film person when it comes to BBM).
But, I do think it's also important that we all draw our own conclusions or work-out interpretations of the scenario for ourselves. So, my secondary answer is that I have a hunch that he probably was murdered (but again, I have no real basis for proving this... it just seems like it might be the case to me). I say this because, from the very beginning, I find the accident scenario - in the way that Lureen describes it - a little too far-fetched.
In the end, all we know is what Lureen says and what Ennis thinks. I do sort of think Ennis's immediate response (of immediately jumping to the murder scenario) is a little bit a sign of his "paranoia" or a sign of how extensive and powerful his fears about violence have become. I think the murder vs. accident question has a lot to do with whether the audience believes Ennis knew Jack better of if Lureen knew Jack better. And by "knew" I mean understand both the circumstances of his daily/practical life vs. knew his inner-self.