But we saw logo of ranger on the jacket of criminal.If you suspecting the police officer then how did he know the secret sign of Hyun joo and yi gang?

When da won killed by the murderer ,He also manipulated their secret sign.

I think it's Kim sol.When they first discover their secret sign he was there.When da won was killed he was also on the mountain that day.He also relate to the Black valley

I d say it's actually Woong Soon, the policeman, because even in earlier episodes he denied or acted as if he was clueless, when someone asked him something.

 Kahl:

I have the same feeling as you. After yesterday's episode, I am 90% sure it's him! They could not find out why the bus took this exact road and bridge. As a police officer, he was able to plot all of those murders. He has access to all of the info and most people would not be suspicious of him just because of his profession. And which gives me goose pimple is his smile on the preview for next ep... It was pure evil.


EDIT after 14th ep: Jesus... Maybe it's Kim Sol after all?!

There were actually 2 culprits Lee Se Wook and Kim Sol, ypu got it right!