Has nobody noticed at the coal mining site? (Possible spoilers)

I just watched this film for the first time and loved it. I don't think you're supposed to know, just as with the film Zodiac, who the killer is as its based on a real-life unsolved case. But the film gives a lot of hints as to who it might be.

After the chase scene with the detectives and the pervert jerking off at the crime scene, they come across the coal mining ground (assuming that's what it was at least, a construction site of some sort). Park eventually spots the man with the red underwear and they round up the workers in the vicinity.

As the camera pans across the group, the first man without a face mask on is the same man who Park questioned at the beginning of the film (the man who pushed the button on the typewriter for him). It's the same man.

However, does this mean he's the killer? I've also heard people discuss the maintenence worker during the interrogation and I'm sure there are bound to be many more ideas as to who the killer is. But I think by noticing that the man was the same person from the beginning of the film, and considering the ending where the little girl says the man's face was "ordinary" and "plain", I think Bong is trying to imply that the killer could be anybody. He's simply someone who just hides in plain sight, shuffles through a crowd, hiding in the background.

It makes me wonder if the rest of the people Park questioned in the beginning are littered throughout the film. I always thought it was odd that Park didn't recognize the man when he was at the construction site.

Anyone else notice this?

Last Film Seen:
Memories of Murder - 9/10
http://tinyurl.com/786893y