In Chaos: Charles Manson, the CIA, and the Secret History of the Sixties (Little, Brown, June), O’Neill challenges the accepted narrative about the Tate/LaBianca murders 50 years ago.

Vincent Bugliosi prosecuted the case in 1970, and you interviewed him a few times. Was he supportive?

I met him in April 1999 and was initially awestruck. He gave me so much access, six hours that first day, and several phone conversations after that. But once I found out he was monitoring me—checking in with people I was interviewing—I started backing away. I was looking into his conduct at the trial, which really upset him. Stephen Kay, his coprosecutor, was physically threatened by Vince. I started hearing from cops and attorneys that he wasn’t mentally well. Buck Compton, a chief deputy DA said giving the case to Bugliosi was the biggest mistake of his life, because he created a monster.

With all the information you gathered about Manson and the crimes, how did you organize the book?

In the first few chapters, I had to lay out the official narrative that Bugliosi concocted in his book Helter Skelter, and in the trial, and then I gradually took it apart. That was one of the biggest obstacles of the book, to make it interesting—the motives, why the people were killed, were they targeted or not. I did that first, so that everything I revealed later had a context. I interviewed 500 people, critical witnesses and suspects. Several had never talked to the press or media before. The new information will be shocking, even to those familiar with the case.

What role did Dan Piepenbering, your collaborator, play?

The first manuscript was 120,000 words and only had a quarter of the story! I needed a collaborator to help me distill the most important stuff, because I’d lost perspective, and my faith in myself wavered. Before Prince died, Dan collaborated on his memoir. I figured if he was good enough for Prince, he was good enough for me. I was apprehensive, because Dan wasn’t even alive when the murders happened. But he’s really smart, really intuitive, and a great writer. I concentrated on the narrative, and Dan went back in time and worked on the big picture. We used lots of what I had in the proposal, and it turned out to be a real mélange.

What do you think really happened on Aug. 9, 1969?

I don’t want to say until I can say with certainty. If I speculate, I might undermine the stuff that I also think is possible. One person who might really know is Tex Watson, a member of the Manson family serving a life sentence, but he’s not talking.