Hulton Archive / Getty American cult leader and murderer Charles Manson appears in a police mug shot

Few killers have ever fascinated and horrified the public as much as Charles Manson. Forty years ago this week, on Aug. 9, 1969, Manson's hippie-styled followers, the Manson Family, murdered actress Sharon Tate, wife of director Roman Polanski, and four other visitors to her Los Angeles estate. The murders were gruesome, with numerous stabbings and shootings, and PIG written on the wall in blood. The next night, the group brutally murdered a married couple, Leno and Rosemary LaBianca, in their Los Angeles home, again leaving messages in blood all over the house. The trial took an unprecedented nine months, hypnotizing the nation. The determined prosecutor, Vincent Bugliosi, won death sentences for Manson and his band. (California later struck down the death penalty, and the sentences were commuted to life. Manson, now 74, remains on death row at Corcoran State Prison near Fresno, Calif.) Afterward, Bugliosi wrote Helter Skelter (W.W. Norton), a spellbinding account of the Manson murders and trial that became the No. 1 true-crime best seller of all time. TIME senior reporter Andrea Sachs reached the author at his home in California. (Read a 1970 TIME article on Manson's trial.)

What is the meaning of the title of your book?

Helter-skelter was the motive for the murders. Manson borrowed that term from a Beatles song on the White Album. In England, helter-skelter is a playground ride. To Manson, helter-skelter meant a war between whites and blacks that the Beatles were in favor of. When the album first came out, in December of '68, he got a copy, and he came racing back to the ranch all excited and said, "The Beatles are telling it like it is! The s___ is coming down!" It was this war that he felt he could ignite by killing white people and blaming black militants, this war called helter-skelter.

Why the enduring public fascination with Manson?

I'm not aware of any other murder case in American history, other than the assassination of President Kennedy, where [anniversaries] are marked by television specials, news reports and articles. Before the murders, no one associated hippies with violence and murder, just drugs, peace, free love, etc. Then the Manson Family comes along, looking like hippies, but what they were all about was murder. That was their religion, their credo. That shocked a lot of people and definitely hurt the counterculture movement. I think the main reason for the continuing fascination is that the murder case is almost assuredly the most bizarre mass-murder case in the recorded annals of American crime. The Beatles were somehow involved. The killers were young kids from average American homes.

Was he mentally ill or just purely evil?

His moral values were completely twisted and warped, but let's not confuse that with insanity. He was crazy in the way that Hitler was crazy. In fact, Hitler was Manson's greatest hero  he spoke about Hitler all the time. He said that Hitler had the right answer for everything, that he was a tuned-in guy. So he's not crazy  he's an evil, sophisticated con man. We're talking about evil here, as opposed to mental illness. Manson wanted to kill as many people as he could.

You say his followers started out from normal families. Was he really able to warp them to that degree?

I think that the ones that killed for him, even though they came from fairly good homes, had a lot of hostility inside of them, and Manson was the catalyst that brought that hostility to the surface. At the trial, I distinguished those who killed for Manson, and [who] did it with relish, from those who would die for him but would not kill for him. I will stipulate that these murders would have never taken place if it had not been for Charles Manson, but they also wouldn't have taken place if these people never wanted to commit murder.

Let's talk about your role as prosecutor. What was that trial like? It was the longest trial that had ever been held up to that point.

I was honored that the DA had enough confidence to assign a case of that magnitude and complexity to me. I worked on it around the clock, seven days a week, sometimes 80 or 90 hours. The trial was almost as bizarre as the murders themselves. One day, Manson got a hold of a sharp pencil and, from a standing position, he leaps over the counsel table and starts to approach the judge, and of course the bailiffs immediately tackle him, and he shouted out to the judge, "In the name of Christian justice, I want to chop off your head." The judge started carrying a .38-caliber revolver under his robe in court. One of the defense attorneys vanished from the face of the earth during the trial and turned up dead.

What was it like when Manson testified?

He took the stand, but it was outside the presence of the jury. If you want to call it testimony, he was under oath and for an hour or so kind of mesmerized everyone. He just rambled on discursively. When it came time for cross-examination, I asked him a couple of sarcastic questions. Afterwards, the judge asked me why I didn't cross-examine Manson, and I said, "Judge, the jury was upstairs. I don't want to give him a dry run."

Are you sorry he wasn't executed?

I don't use the word sorry, but he should have been executed, and I told the jury, if this was not a proper case for the imposition of the death penalty, then no case ever would be. Manson did not deserve to live.

Do you think he is still dangerous?

To my knowledge, there is no Manson Family out there today. Everyone has renounced him. The only two that tried to keep the flame alive were Squeaky [Fromme], who tried to assassinate President Ford, and Sandra Good. However, he's got more supporters and sympathizers now than he ever did. He doesn't have people out there right now like his Family whom he could tell to go out and kill people, but he does have young people who view him like a glorious outlaw.