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When an acclaimed director's new documentary about Scientology made its premiere at the exclusive Sundance Film Festival this winter, the church was watching closely, according to one of the producers.

Very closely.

"Two of our participants were surveilled when they arrived at Salt Lake City Airport," Lawrence Wright told NBC News.

Wright is the author of "Going Clear: Scientology, Hollywood and the Prison of Belief," a 2013 best-seller that dug deep into the religion founded by science-fiction writer L. Ron Hubbard and championed by celebrities like Tom Cruise and John Travolta.

The book was the basis of the documentary by Oscar-winning filmmaker Alex Gibney that airs on HBO on March 29 — marking Hollywood's first major effort to unravel the mysteries of an organization that has thousands of outposts around the world.

By Wright's account, the scrutiny the film's sources experienced at the airport in Utah was nothing new — just part of what he says is ongoing monitoring and harassment by the church against those who have left its star-studded ranks.

"One former member discovered a surveillance camera in a birdhouse across the street. Private investigators follow them. It's an ongoing campaign of trying to intimidate former members and to silence," said Wright, a Pulitzer Prize winner.

A Scientology spokesperson said the church sent investigative reporters for its magazine to Sundance but denies harassing former members. It has branded the film a one-sided pack of lies by "bitter, vengeful apostates" and "admitted perjurers, admitted liars and professional anti-Scientologists."

READ: For the Church of Scientology's full statement on the documentary, click here.

The documentary profiles eight former members of Scientology, including Mark "Marty" Rathbun, who worked for the church for 27 years before he left in 2004.

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In an interview Friday morning on TODAY, Rathbun said he "escaped" from the church after being sentenced to a "prison camp" known as "The Hole" where members were forced to take part in group confessions and violent acts against each other.

Scientology officials say "The Hole" doesn't exist and that he never "escaped" — that he was working at the Scientology spiritual headquarters for a year before leaving the church.

Rathbun also repeated an allegation he makes in the documentary — that actress Nicole Kidman was wiretapped by the church.

"I ordered that it happen on the order of David Miscavige," Rathbun said. The church and Miscavige vehemently deny it. They state that there is absolutely no evidence to corroborate this claim and say that Rathbun only recently leveled the accusations and made it up out of thin air.

Because of his outspoken criticism, Rathbun says, private investigators constantly tail him and high-tech cameras were placed in the woods around his Texas home.

"It's like being hunted game," Rathbun said. "We don't have a normal life and I don't think we ever will."

His wife, Monique, obtained a temporary restraining order almost two years ago that bars the church or its representatives from pestering or monitoring her.

"The defendants have worked around the clock for three years to destroy Mrs. Rathbun," her lawsuit against the church alleges.

"She has been harassed, insulted, surveilled, photographed, videotaped, defamed and humiliated to such a degree as to shock the conscience of any decent law-abiding person. She has been subjected to numerous aggressive attempts to intimidate her."

Scientology denies stalking Rathbun or his wife.

In court papers, the church said a few members who called themselves “Squirrel Busters” filmed the couple because they were making a documentary about his self-styled brand of Scientology.

The church said in a statement that it hired private investigators to look into "Rathbun's appropriation of the Church’s intellectual property, his threat to destroy its copyrights, his exhortations to others to engage in theft of Church materials and property and his public attacks upon Scientology and its officials." It has maintained for years that Rathbun is an excommunicated self-promoting liar who is profiting from his agenda.

While a number of former members have been speaking out against Scientology for years, Wright notes that "Going Clear" is the first time Hollywood has given them a major platform. "The industry has always been frightened," he said.

In response, the church produced an eight-minute video criticizing Gibney, and in 13 other videos like it attempts to discredit his sources.

Wright said that didn't come as a surprise.

"You don't do a story or documentary about Scientology without expecting some kind of retaliation," he said.

The church says it's actually the victim here.

"Wright and Gibney cherry-picked expelled, discredited former Scientologists who would help them advance their propaganda," the church statement said. "What was portrayed as a nonfiction book, and now a film, are both transparent vehicles for vendettas."