The Church of Scientology isn't prepared to sit back and let influential Hollywood types use the medium of satire to poke fun at it.

Just ask Matt Stone and Trey Parker, the creators of South Park. According to a newly leaked email, their private lives have been forensically examined by investigators working for the Church's PR department.

They appear to have ended up in the line of fire in 2006, not long after the broadcast of "Trapped in the Closet", a highly controversial episode of the adult cartoon, which attempted to mock the Church's science fiction-based theology. According to an internal memorandum, Scientology's Office of Special Affairs mounted an investigation into Parker and Stone's circle of close friends, seeking "viable strings that can be pulled" to either discredit the duo, or to persuade them to refrain from satirising the Church in future.

"To find a direct line into Stone and Parker, some of their friends have been identified," reads the email. They included Matthew Prager, a Hollywood screenwriter, actors John Stamos and Rebecca Romjin, and Dave Goodman, a writer who was at college with the pair.

The memo says each of these individuals is being "PRC'd" – a term used by private detectives which refers to the practice of checking every public record available about a particular target, in search of a potential vulnerability.

"There are some strings that will be pulled on the PRC on Stone. Otherwise the special collections [a term apparently referring to data gathered using covert surveillance techniques] will be debugged in order to get some viable strings that can be pulled."

The memo, written on 16 April 2006, was published at the weekend by Marty Rathbun, a former high-ranking Scientology executive who defected five years ago and has since been running a blog critical of the Church. He claims it is part of a large "trove" of hitherto-secret documents detailing the Church's efforts to discredit the South Park creators, which recently found its way into his possession. He has promised to release others.

In interviews yesterday, Mr Rathbun alleged it is common practice for Scientologists to gather sensitive information about their critics. In addition to scouring public records, he claimed Church investigators occasionally examine the rubbish bins of targets.

Stone and Parker have for years used South Park to satirise almost every organised religion, sometimes with awkward consequences. The "Trapped in the Closet" episode, which featured an unflattering portrayal of Tom Cruise, was a case in point: it wasn't broadcast in the UK for several years, because of the strict libel laws. Controversy about the plot – in which Church elders decide South Park character Stan is the re-incarnation of L Ron Hubbard, the founder of Scientology – resulted in it becoming one of the most-watched episodes in the show's history.