The Word of Life Christian Church is examined on People Magazine Investigates: Cults, airing Monday (9 p.m. ET) on Investigation Discovery

Inside the Church Cult That Eventually Drove Parents to Join a 12-Hour Beating of Their Sons

Residents of Chadwicks, New York, had long thought there might be something wrong with the Word of Life Christian Church. Those suspicions were confirmed on the night of Oct. 11, 2015, when two teenage brothers were savagely beaten for a dozen hours inside the church’s sanctuary room after trying to defect.

The vicious attack — at times carried out by the boys’ parents — left 19-year-old Lucas Leonard dead and his then-17-year-old brother, Christopher, severely injured.

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Soon pulled under the harsh scrutiny of a police investigation, the little-known religious sect was revealed as a dangerous cult.

Nearly three years later and Word of Life is no more. Christopher continues to recover from his injuries and relatives tell PEOPLE he recently graduated from high school.

All told, nine people were sent to prison for their involvement in Lucas’ death — among them, the church’s pastor, Tiffanie Irwin, who inherited the group from her father before bending it even further to her will.

Many who’ve followed the case still wonder how Bruce and Deborah Leonard could have been compelled to kick, punch, strike and whip their two sons over 12 torturous hours. The case and the church itself will be featured on Monday night’s episode of People Magazine Investigates: Cults, airing on Investigation Discovery and exclusively previewed above.

As Bruce later said in court, the attack began following a Sunday service when Irwin called his family together and confronted the teenage brothers with allegations of child abuse and witchcraft.

Bruce told a judge the beatings started with a punch and a slap and quickly progressed to electrical cord whippings. The point he said, was so his sons “would understand the hurt that they caused” and to discipline and punish them.

Though the question of child sex abuse came up in court, with Christopher testifying he and Lucas molested the children of several Word of Life Church congregants, investigators have always insisted there was no evidence to indicate they ever harmed any children.

• 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.

Image zoom Christopher Leonard (left) and Lucas Leonard

According to cult expert Rick Ross, the Leonard parents had been brainwashed over time and would have followed any instruction from Irwin, who claimed to be their only reliable path toward salvation.

“As with most cults, they manipulated people like pawns,” Ross previously told PEOPLE. “Some cults are more destructive than others, but from what I have observed and read the destruction wrought by the Irwin family caused families to be disrupted and estranged.”

The Word of Life Christian Church, which operated out of a rundown brick building that once housed a school, was founded in 1987 by Tiffanie Irwin’s late father, Jerry Irwin. The isolated group lost touch with reality not long after Tiffanie assumed control in 2012, when her dad died.

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As the group’s membership dwindled in recent years, the Leonard brothers announced their plans to defect, according to Ross. That’s when Tiffanie accused them of sexually abusing their younger cousins.

“Church leaders believed that the boys leaving would cause others to leave — maybe even their parents, who were pivotal members of the group,” Ross explained. “What happened here had everything to do with the retention of members and the maintenance of control.”

Those that left, Ross posited, “were considered damned. There was no place else to go. The Irwin family wanted people to feel like there was no alternative … that they were the one and only true church in the world.”

After Lucas bled to death inside a nearby hospital, his parents were charged with his killing. Also arrested were Tiffanie, her brothers, Joseph and Daniel, and her mother, Traci.

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

Prosecutors offered plea deals to Bruce, who was sentenced to 10 years on two counts of assault, and Deborah, who was sentenced to five years on the same charges as well as a charge of manslaughter.

Nearly all of the other suspects admitted some responsibility in the attack.

Tiffanie avoided a murder charge by pleading guilty to manslaughter and assault and was sentenced to 12 years. Brother Joseph also pleaded guilty to first-degree gang assault and second-degree assault and was sentenced to eight years in prison.

Church member David Morey and his mother, Linda, both pleaded guilty to two counts of second-degree assault and were sentenced to five years.

Tiffanie’s mother, Traci, and her son, Daniel, pleaded guilty to unlawful imprisonment and were both sentenced to two years.

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Sarah Ferguson, the Leonard brothers’ half-sister, was the only defendant involved in the attack to decline a plea deal. In July 2016, she was convicted by a jury of manslaughter and assault — but was cleared of murder. She was sentenced to 25 years in prison.

Nearly three years since the excruciating assault and, his family says, Christopher still grapples with his wounds from that day. Though he is “well” and has many friends, “Recovery is not even in the cards for a very long time for any of us,” sister Kristel Leonard tells PEOPLE.

“And it really is not about the parents,” she says. “It is because of the cult experience.”

Christopher’s contact with the adult members of the church “has been very minimal to none,” Kristel says.

“I think he has made a conscious choice to close that horrid chapter,” she says, “and move on.”