it s my dad and the deputy, and, yeah, they come in. it s kind of a shock to see, you know, when you re expecting somebody, to see strangers coming in instead. reporter: what do you think when you see that? not a whole lot of time to think, honestly. you kind of go into shutdown mode. reporter: alex recalled his dad seemed like he was in shock. he was very disheveled, had been crying a lot. and then he, you know, gave me a big hug, and that was kind of how it broke. reporter: i can t even imagine getting that news, when you ve just thought your parents were going for a quick hike. just kind of disbelief, really. the whole thing just seemed so surreal. reporter: then father and son were shuttled to the sheriff s department where fred could file a report about the accident. i remember them saying, you take fred, i ll take alex. reporter: sheriff bruce was at the station to meet them. my initial reaction to fred was, he was a victim, a grieving husband. reporter:
reporter: time went by and the muellers didn t hear anything from the colorado investigators. the kids assumed the sheriff and his team had finally decided to believe their dad, too. we didn t hear from them for three years after that. and so i thought that i just thought it was done. yeah, we didn t i guess they finished up their crossed their paperwork or whatever they needed to do. t s and and that it was over. reporter: it wasn t over. i am an open book. reporter: in fact, after fred s interview with the cbi, investigators kept working. what drove them was their theory about a possible struggle, and that curious lack of injuries on leslie s body. but the sheriff knew they needed more, and he put that photo of leslie behind his desk. she was my constant reminder. and that photo was what was the nudge that kept me going on this case. reporter: investigators decided to zero in on one aspect of fred s story.