More of a documentary than a thriller like the Director's earlier movie Seven -- and after a very long 2 and a half hours it gives a best guess as to who the writer thinks the killer was - and its one of the poorest, anti-climactic endings you'll ever see.
After the genuinely-scary basement scene, Jake discounts that guy as the killer and concentrates on the man who looks like Kevin from The Office: An American Workplace. Obsessed with the case, his family leave him and he lives a paranoid, lonely life in his studio apartment tormented by calls that we assume are from the killer.
To get some closure, he walks into Kevin's shop while he is serving a customer and just stands there. They exchange a stare, and then he leaves. That's it. He can live his life as he has now 'met' the killer.
At the end it tells us that Jake received calls on a regular basis - but they all ended when Kevin the Killer died from a long-term illness.