BOULDER, Colo. -- He may plead guilty Thursday, but even for the two Boulder County district attorneys who have worked on his case for years, Scott Kimball still has secrets.

"I think we would be hard-pressed to think that we caught him on everything he did," said Katharina Booth.

Booth is one of two chief deputy district attorneys who have pursued Kimball, a Colorado native, for more than five years, while maintaining a full slate of other criminal cases.

Starting as a child abuse case out of Lafayette in 2004, the women would learn enough about Kimball to suspect him in other crimes. But they would quickly realize that nothing about their subject came easily or free.

Just piecing together his lengthy, dispersed criminal history took six to nine months.

"We usually get a printout. And we can look at it. And in an excessive amount of time, it might be 30 minutes to figure it out," said prosecutor Amy Okubo.

"There were many times that it felt insurmountable," Booth said.

On Thursday, the multi-state, multi-body crime spree ends for Kimball. He is facing two choices go to trial on two second-degree murder charges or plead guilty.

"It's like, pinch me! I can't believe this is happening," Okubo said. "Because there were meetings, and meetings, and meetings where we asked, 'Can we ever achieve what we want to achieve?' And frankly, the answer quite often was, 'It's too big a thing to deal with.'"

From check forgery and other white-collar, financial crimes, they would learn enough about the avid hunter to suspect he could be capable of murder.

"He's a master manipulator," Okubo said. "There were always, 'Are you kidding me?' moments."

Indeed, despite convictions in Colorado, Montana, Alaska and Washington state, Kimball convinced the FBI to use him as an informant in 2002. Soon after, people around him in Colorado started disappearing.

Three women disappeared in 2003 and Kimball's uncle, Terry Kimball, disappeared in 2004.

Kaysi McLeod, LeAnn Emry, Jennifer Marcum and Terry Kimball are all believed by prosecutors and investigators to be Kimball's murder victims.

But noting Kimball's travels to Nevada, Utah and California, both Okubo and Booth believe Kimball may have killed elsewhere.

They now say Kimball was sexually abused as a child and developed a bondage and rape fetish as an adult. A scar on his forehead came from a failed attempt to commit suicide, Booth said.

If Kimball pleads guilty to second-degree murder, he could earn up to 48 years in prison for each charge.

He is already in prison for more than 50 years, with 48 years owed to the state based on theft charges Booth and Okubo helped convict him on, as well as 70 months in federal prison for weapons charges.

In their entire careers though, both district attorneys said Kimball has been the most challenging and unusual criminal they've ever tracked.

"I think this is a one-of-a-kind case. And the magnitude of it ... the pressure of what we could do to help the families ... it took a personal toll on us," Booth said.