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Prophet’s Prey starts off as a documentary and ends up feeling like a horror story. Amy Berg’s film about Warren Jeffs, the leader of the Fundamentalist Church of Jesus Christ of Latter-Day Saints, is a portrait of a loathsome man who, though imprisoned, still maintains power over his flock. His message to his followers: If he’s still locked up, then they aren’t praying hard enough.

That, by far, is the least of Jeffs’ manipulations. The film starts out with an animated history of the Church of Jesus Christ of Latter-Day Saints—more commonly known as the Mormon Church—which the Fundamentalist Church of Jesus Christ of Latter-Day Saints broke away from more than a century ago over the original church’s renouncement of polygamy.

Warren’s father, Rulon Jeffs, became president of the church in 1986, and remained so until his suspicious death in 2002. Warren had already had influence as a schoolteacher by then (starting as soon as he finished high school at 17) and acted quickly when his father died: Rulon’s dozens of wives were to continue behaving as if he were in the next room, and they were hands-off to the rest of the male congregation.

Warren, though, would go on to marry each of them—including the teenagers he forcibly wed to his father before his death—and soon many more, allegedly around 70.

Berg (2012’s West of Memphis) lets two men tell the bulk of the story. Sam Brower is a private investigator and the author of Prophet’s Prey (though he doesn’t share a screenwriting credit with Berg). Into The Wild scribe Jon Krakauer, who also provided commentary in the recent Meru, is the author of Under the Banner of Heaven, an “investigative nonfiction” book that sheds light on both the traditional and the fundamentalist churches.

Both Krakauer and Brower have let FLDS become their obsession, with a sense of duty to, if not disband it, at least lead authorities to proof of the church’s many misdeeds. Among those misdeeds are not only polygamy but fraudulent businesses and a generally cultlike atmosphere in which all sense of fun or recreation—from TV to Internet to fishing to pets—is forbidden.

And then there’s the rape.

Until this point in the film—even when it’s brought to light that Jeffs was molesting children—you’ll shake your head. But it’s when he’s apprehended on charges of rape—technically statutory, though that qualifier isn’t really needed—you’ll want to vomit. Footage exists of some of the court proceedings, including the repugnant photos of Jeffs kissing his child brides. It’s the audio tape, however, of Jeffs raping a 12-year-old (virgin) girl that will really make you put down the popcorn, if not outright leave the theater.

Berg’s decision to include this repulsive evidence is controversial—we already know the man is a monster; why drag a specific, and named, girl into the argument? It’s tough to bear witness to, and thankfully it doesn’t go on long.

Jeffs is also shown being interviewed while incarcerated (he answers “Fifth Amendment” to most questions). Throughout the film, Berg inserts recordings of his sermons that will chill your spine, not necessarily for his “I am God”-like preaching, but merely the sound of his voice—soft, even sleepy, yet mechanical. Remember HAL? Vocal twins.

Needless to say, if Berg sought to demonize Jeffs, she does the job well. Krakauer remarks that if he were a religious man, he’d pray for Jeffs’ demise. Instead, he offers what many viewers may already be thinking: “I hope that fucker dies right now.”

Prophet’s Prey opens Friday at the Angelika Pop-Up.