A very pure and specific dialect is rarely used in manga. Why not? Because the vast majority of readers would not understand it then. (The Tokyo dialect might be the only exception.)
The cop's speech looks Touhoku-ish for sure, but if you looked closely, except for the use of 「わらす」 (= わらし), almost all of the dialectal elements used are the voicing of the consonants that are pronounced unvoiced in "standard" Japanese. I am referring to:
I have lived in Nagoya and Tokyo all of my life. If I can understand the cop's lines if I just mentally got rid of all the ゛'s (the dots), then I know that that is not a full-on dialect.