On the second day I spent with the Manson Bloggers, we found a tongue hanging from a tree. This was in the northwestern fringes of Los Angeles County, the half-wild, half-suburban part of the city that the Manson Family once called home. These days, most of the land is owned by the state and nearby there is a church; on top of a hill, a ten-foot cross looms in right-angled judgment. The Manson Bloggers did not seem to notice the cross, because they had another mission in mind: finding the Manson Tree, a gnarled oak that’s notable because Charles Manson used to perch in its crook and strum the guitar.

We had to scramble over a highway railing to reach the old oak. As we got close, I saw that some previous visitor had thrown a white rope over one of the tree’s branches. Something was dangling from the rope—a sweet potato, I thought. Or some sort of lumpy, orangish doll. The Manson Bloggers knew better. “It’s a cow’s tongue,” Deb said. She was right. Up close, it was unmistakable, a length of moist muscle, obscene and obscurely violent. The tongue was covered with rainbow sprinkles, the kind you’d put on a child’s scoop of vanilla ice cream. One end of the white rope was tied around the tongue’s root, where it had once been attached to the back of the cow’s throat. The other end of the rope was tied around a bottle of fish-oil pills. There was one AA battery inside the fish-oil bottle. On the ground was a crumpled-up shopping bag from H&M.

The Manson Bloggers and I stared for a moment in mute wonder. The tongue, the rope, the sprinkles, the fish-oil bottle, the battery, the H&M bag: it all spoke to some inexplicable ritual, a dark magic that somehow brought together cult murder, fast fashion, and nutritional supplements. I’ll be honest, I was spooked. The bloggers took it all in stride. Maybe their world accommodated more strangeness than mine. Or perhaps they were just used to finding messages of violence in unexpected places.

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One year earlier, in the first throes of my renewed interest in the Manson Family, I had taken the Helter Skelter bus tour operated by Dearly Departed, an outfit specializing in Los Angeles death and murder sites. It sells out nearly every weekend, which is surprising given that there’s actually not much to see anymore. Scott Michaels, Dearly Departed’s founder and main guide, set the tone with a custom soundtrack of hits from 1969 (“In the Year 2525,” “Hair”), cozy oldies made newly spooky by our proximity to death. He also included extensive multimedia add-ons,...