Does the Erroll Morris book leave the impression that Janet Malcolm has never bothered to confront the possibility that MacDonald may be innocent? If so, that's unfair. From a 1997 New Republic article, by Ruth Shalit:
Journalists, armed with the mountain of FOIA documents Malcolm found so understandably oppressive, are asking the unsettling question: What if MacDonald is innocent, after all? Errol Morris, who exposed a famous miscarriage of justice in his award-winning documentary, The Thin Blue Line, says he is now trying to get money to make a movie about the MacDonald murders. "One gets the impression reading Fatal Vision that there was an overwhelming case against Jeffrey MacDonald. In fact, that is just not true," Morris told me. "It's the quintessential postmodern nightmare--a man who is in jail because everyone thinks they know the story, when the story, for the most part, has been framed by a book and a television movie, rather than a careful examination of the underlying evidence." Janet Malcolm has now brought herself to look at the FOIA documents as well. "The more I read about the case," she told me, "the more I move toward the view that he could very well be innocent."
(Yes, it's Ruth Shalit, but it's a good piece...)