New York ? Producers of a made-for-TV movie about the BTK killer were preparing a story about a community terrorized and investigators frustrated by a mystery man – until Dennis Rader was caught.

It forced them to rewrite on the fly and rush to make deadlines. The final scene was shot less than a month ago and the movie was still being edited this week – even though “The Hunt for the BTK Killer” is scheduled to air 8 p.m. CDT Sunday on CBS.

Judith Verno, the film’s executive producer, said she optioned the rights to Robert Beattie’s book on the BTK killer last year when the then-unknown Rader began communicating with authorities after years of silence.

The movie she was shopping to networks was to focus on the search, and its best-known actors (Michael Michele of “ER” and Robert Forster) were composite characters representing the investigators who had been trying to crack the case for decades.

Rader, a city worker who gave himself the nickname that stood for “Bind, Torture, Kill,” was arrested in February. He pleaded guilty and was sentenced to multiple life sentences in August for 10 killings between 1974 and 1991.

His arrest was both a boon and bane to the filmmakers. CBS swiftly bought rights to the film after Rader was taken into custody, but it forced major changes.

“It’s a very strange experience to be working on a movie while it is unfolding, especially when you’ve been working on it for a year and a half,” said Verno, whose TV movies include last year’s “The Perfect Husband: The Laci Peterson Story.”

She didn’t want to change the film’s focus on the investigation. But obviously Rader’s capture changed the story, and details provided in his interrogations and court appearances provided writers with insight into the mind of a serial killer.

“We found out that a guy you knew was unbelievably sick and weird was even weirder and sicker than you might think,” said actor Gregg Henry, who portrays Rader.

Both Verno and Henry were struck by Rader’s creepy confidence, his sense that he was smarter than those chasing him. From studying clips of Rader’s appearances, Henry – whose films include “Ballistic: Ecks vs. Sever” and “Body Double” – picked up vocal traits and the way the killer’s carefully constructed mask of self-control was undermined by tension you could read around his mouth.

The movie was rewritten to include a much lengthier scene of Rader’s interrogation, Henry said.