This was a great "mystery" to me too after seeing the movie for the very first time...(<---like WAAAAAAAAAaaaaaaaaayyy back then! Ha, ha, ha!

Since I bought the dvd, I think I must have watched it about 50 times by now....! :o

)
Aaaaaaaaaaaany-waaaaaaaaays....it bothered me to no end, so I did some intensive research on this particular subject, reading and listening to everything that I could find on the internet about it. And it's curious to see, that while the views differ amongst the general (theatre) 'viewers' of the film (including a good number of the film critics!), every single one of the people involved in the actual MAKING of the movie, ranging from Ang Lee, Anne Hathaway, McMurtry and Ossana, Jake Gyllenhaal, Annie Proulx etc. etc.~~ all the way down the ladder including make-up men, set dressers etc.
all believe that Lureen knows. This is what I could find.
Funny though as: no one actually dares to come right out and say: "Jack was murdered"...everyone always just says something like: "It is IN MY BELIEF, rather apparent that Lureen knows..."
After doing this research, my heart was stilled, and I was finally in peace again. I personally, decided that the viewpoints from the persons involved with the actual making of the movie, held more weight for me than the opinions of the few "others". (They lived with, studied meticulously and worked with Proulx's material for a long time while putting the film together~needless to say: much, much longer than I had.) So I chose to accept and embrace their professional viewpoint as also my own. (It wasn't very difficult to do, as this was also my opinion anyway, even before I conducted my mini-search on the net & after having only seen the movie once...) But by doing this, I could finally watch the movie again, interpreting and understanding things in an appropriate way.
It is my opinion, that the movie was filmed in this way, not blatantly revealing by spelling it out in giant "black and white" letters, to simply stay inherently true to Annie Proulx's writing/short story (it was written this way...). And as is often the case in great literature, a small amount of ambiguity, the so-called: "leaving-it-up-to-the-reader's-discretion" is always a desirable thing. Very attractive from the reader's point of view, because it subtly binds him to the material, stealthily drawing him into the story, while graciously offering him his own personal "input" into the already existing piece of art...
McMurtry and Ossana's well-thought out and ingeniously written (perfect!) screenplay combined with Ang Lee's very skillfull manner of filming of this rather difficult aspect from Proulx's short story is truly marvelous. It is more than likely that in less capable hands, it might have delivered a film of mediocre quality. But they present us instead, with a truly great piece of cinema. The best movie I have seen in a loooooooooong time...!
And the proof of this is: merely the fact that this had taken a hold of me, enticing me so strongly in it's 'grip'...and actually FORCING me out of my usual laid-back and rather pitiful apathetic "couch potato" manner of film viewing ("Ok, ok, start the damn film...& lemme see what you got...while I lay back, suck up my diet coke through a straw and crunch on some mundane kernels of corn)~~making me actively do my research for a/the "truth".... And believe me bud, that is NOT an easy thing to do!

Whew! Even after like 50 viewings, I am
still emotionally and physically drained at the end of the movie... I must be a major sufferer of PBM syndrome...
sam