In 1970, Colette MacDonald and her two daughters were stabbed to death in a savage frenzy, setting in motion America's longest-running murder trial. After decades of legal wrangles, her husband, a military doctor, remains in jail for the crime. We spoke to Errol Morris, an Oscar-winning film-maker who has written a book about the case which reaches some startling conclusions…

The Oscar-winning film-maker Errol Morris made his name in 1988 with The Thin Blue Line, a bravura piece of documentary-making that gained the release from prison of an innocent man who had been on death row. But although he spent several years working on that investigation, it's not this crime that has maintained the most insistent hold on his intellect and imagination. That prize goes to one committed on 17 February 1970 at Fort Bragg in North Carolina.

He is not alone in his obsession. The killings that took place in the early hours...