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.”

Scott Kimball looks up while the charges and sentencing requirements are read during his hearing at the Boulder County Justice Center in Boulder, Colorado, Thursday, Oct. 8, 2009. Kimball pleaded guilty Thursday in the deaths of four people and was sentenced to 70 years in prison.  (AP Photo\Mark Leffingwell)

In 2006, pressure from victims’ family members forced the FBI to begin investigating its own informant. 

Former FBI Special Agent Jonny Grusing speaks with Fox News Digital at CrimeCon 2025 in Denver on Sept. 6, 2025.  (Peter D’Abrosca/Fox News Digital)

“And that’s when two dads came to the FBI office to talk to my boss and say, not only was Scott responsible for Jennifer’s disappearance, but another girl named Kayci was last with Scott, and that reporting was in the case file,” said Grusing. “But Scott had mastered such that, again, he enjoyed the game, and it was like leaving little breadcrumbs to say, ‘I’m so good at this, I can tell you about these homicides, and you’ll never know I’m doing them.'”

In March of 2006, Kimball was arrested in California on fraud-related charges. 

While he was in jail, the FBI built a case against him, and in 2009, he was officially charged with the murders. 

But the killer’s manipulation of the FBI continued for years, as Grusing and others tried to get him to give up the locations of his victims’ remains. 

“We knew he was manipulating us, pointing us in different directions, but he’s the only one who knew what happened to them,” said Grusing. “So even though it was painful to be in front of him and let him win all the time, as long as he thought he was squirming, he would always talk. So that’s how I stayed with this man for 15 years.”

Eventually, McLeod and Emry’s remains were recovered. 

At one point during Grusing’s decade-and-a-half quest to get justice for more of Kimball’s victims, the killer asked him why he had never been named like other serial killers. 

Grusing asked what name Kimball would give himself.

“He said, ‘The Opportunity Killer, because I just kill people when I have the opportunity.’”

Marcum’s remains have never been found.