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DENVER – A former FBI agent says a serial killer who doubled as an FBI informant for years manipulated agents into believing he was helping them, all the while preying on unsuspecting victims.

Scott Kimball will likely spend the rest of his life in a federal prison in Colorado after he was sentenced to 70 years in 2009. He pleaded guilty to killing four people between 2003 and 2004, and his number of victims could be much higher, according to former FBI Special Agent Jonny Grusing.

"He made a game out of tricking the FBI," Grusing told Fox News Digital, adding that Kimball's case was unprecedented. "As long as he won the game in front of him, that’s all that mattered."

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