Lucas Leonard was beaten to death after he and his brother announced plans to leave the Word of Life Christian Church in upstate New York

WATCH: Church Members Beat Teen So Brutally Doctors Thought He'd Been Shot — and His Parents Joined in

Lucas Leonard’s jeans were soaked with blood when he was brought to the hospital, and doctors initially thought the 19-year-old had been shot.

The reality was far more disturbing: Lucas and his 17-year-old brother, Christopher, had been savagely beaten over a 12-hour period by members of the reclusive Word of Life Christian Church in upstate New York. Lucas died from his wounds.

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Among the assailants were the teens’ parents and the pastor of Word of Life, which is featured on of tonight’s episode of People Magazine Investigates: Cults, airing at 9 p.m. ET on Investigation Discovery.

“Someone dropped a person off with a gunshot wound,” an emergency worker is heard saying in the episode, an exclusive preview of which is shown above.

The episode chronicles Word of Life’s transformation from a small bible study group into an abusive church that turned members against their parents and each other, controlling every aspect of their lives.

“You stupid idiot! How stupid can you be anyway?” the church’s founding pastor, Jerry Irwin, says in the above clip.

Image zoom The Word of Life Christian Church building in New York Mike Groll/AP/REX/Shutterstock

• For more on the Word of Life Christian Church and the death of Lucas Leonard, watch Monday’s People Magazine Investigates: Cults at 9 p.m. ET on Investigation Discovery.

Lucas and Christopher were beaten on Oct. 11, 2015, after they announced their plans to leave the church. The church’s pastor, Tiffanie Irwin, had escalated her control over church congregants over time as church membership dwindled.

“Tiffanie Irwin seemed to be setting up this sadistic atmosphere where the violence just got more and more intense,” PEOPLE Senior Editor Alicia Dennis says in the clip.

Speaking about the teens’ parents, Dennis adds, “They let a church take over so much of their lives that they participated in the murder of their son.”

Image zoom Christopher (left) and Lucas Leonard

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In Monday’s episode, viewers will get their first look at interrogation videos captured immediately after the arrests of parents Bruce and Deborah Leonard.

The couple, while obviously concerned for the condition of Lucas, seemed reluctant to discuss the location of the fatal assault (the church sanctuary) or who was involved.

Image zoom Deborah (left) and Bruce Leonard AP

Nine members of the church were later sentenced for their roles in the attack. Lucas bled to death on a hospital gurney. Christopher survived but is still recovering from his injuries three years later.

In the exclusive clip, Chris Harris, the PEOPLE Senior True Crime reporter who investigated the church, says, “I can’t even imagine that kind of pain.”