First of all, where was he supposed to be living? I know it wasn't western Kansas. So they drove hundreds of miles to question Perry's father, only for him to do almost all the talking for about 10 minutes and then just leave? Did they want to know more than just his whereabouts, then figure they were wasting time listening to the man talk about he and Perry's life in Alaska? Possible plot hole?
shareThe real-life interview certainly would have been longer than shown, but the one depicted in the movie was presumably condensed for time.
share1). They don't say where the father was living. But even if the father lived far away, they were policemen investigating a murder. They would travel any amount of distance to solve a murder. Plus -- as evidenced by the diner scene ("My wife can't piss alone anymore. Why haven't you found the killers yet) as well as the group of reporters who bombard the officer with questions -- the police were under a lot of public pressure to find the killers. So, again, they had a reason to travel long-distance to see Perry's dad.
2). They left after 10 minutes because they quickly sensed that the dad wasn't all there in the head. Plus, the dad already said that he hadn't seen Perry in 5-6 years. He didn't even know Perry was out on parole. So they also figured the father knew nothing useful.