A Lingering Question About The Red Riding Trilogy

Okay, first off - Spoiler Alert. I know spoiling is basically all I do here at the Castle, but the Red Riding movies were largely wonderful, and if you don't want a major plot point spoiled, feel free to just skip ahead to the next article, and check this thing out once you've seen this one.

So, getting to the point, when the third film wraps up, basically every question has been answered, with just one glaring omission.

This is a corrupt cop. Later in the film, he'll be willing to spill the beans about the corruption that permeates every aspect of the police department. Unfortunately, instead of just telling him the serious information, he demands to see the main character in person, and since someone untrustworthy was listening on the other line, he finds himself in this situation just a little while later:

That's him having been tortured to death with a drill. The tape is an audio recording of the torture. Also, his daughter was stabbed to death. Here's the thing - the main character gets from his hotel to the cop's house in considerably less than an hour, which means the killer had to get there and start the torture almost immediately. Which raises the question - just who killed the corrupt cop?

It's important to note, at this point, that every other mystery in the film series has a solution - in the case of every murder we discover who killed them, why, and when. All except for the corrupt cop.

There's only one thing that even counts as a clue, and that's this scene from the final film:

That's the film's killer, a child-raping priest, about to murder someone with a drill. This scene and the murder of the corrupt cop are the only times that drills appear in the film, so it's only natural to assume there's some connection between the two.

This doesn't really make sense as an option, though. While the cops are fully aware that the priest is, at the very least involved in serial killing, there's no reason to believe they'd assign him to an important job like this, especially when they're fine with killing people themselves - and are working on an incredibly short timetable.

So if the priest didn't torture the corrupt cop to death and murder his child, who did, and why doesn't the film let us know? Did I miss something? Is it made clear in the novel, but not in the film?

If anyone reading this knows the answer, please let me know in the comments.