Based on this video interview with Arthur Miller, he appears to believe she committed suicide. However, there's no definite proof that she did and I think Donald Spoto makes a good case that it was an accidental overdose.
But then again, Miller might not be the most reliable when it comes to Marilyn Monroe. In the interview (at minute 6:55) he says of The Misfits, filmed in 1960, that it was "the first dramatic role she ever tried to do." Apparently Miller was unaware of, or discounted "Don't Bother to Knock" "Niagara" (which I just saw this past weekend) which were indisputably dramatic roles. And even "Bus Stop" was a pretty dramatic role. And what's more, they were better roles than what Miller gave Monroe in Misfits, which I talk about here. I can't imagine how Miller could say that.
