The Manson family murder spree of 1969 claimed seven victims, most famously Sharon Tate, the Hollywood flower child whose tragic story is told yet again in Quentin Tarantino’s new film, “Once Upon a Time ... in Hollywood.”

But the murders also altered countless other lives. Consider Tom O’Neill, an entertainment journalist, who in 1999 accepted a three-month assignment from Premiere magazine to write about how the murders changed Hollywood.

He missed that deadline — by 20 years.

As the article grew into an obsession, Mr. O’Neill tracked down enough faded Hollywood luminaries, intelligence operatives, mobsters, drug traffickers and former cult members to, well, fill a Tarantino movie.

Along the way, he came to doubt the accepted narrative of the case as presented by “Helter Skelter,” the 1974 book by Vincent Bugliosi, the lead prosecutor in the case , that Charles Manson was deluded by messianic visions and ordered his drug-addled minions to slaughter rich Los Angeles denizens to spur an apocalyptic race war.