I don't know who the murderer is on The Killing - I suppose we could ask someone in Denmark, since AMC's show is based on a Danish series. But it's more fun to keep guessing, so here's how I've narrowed it down.
The plot took a timely yet improbable twist last week when Linden and Holder followed a lead from a mosque to a meat warehouse, only to be overtaken by FBI agents investigating the same location. Terrorism? As you see in the preview below, Linden thinks the connection to Rosie is unlike - I agree, but only because this surprise turn comes so late in the series it would feel like cheating.
But I've thought for a while that the murderer has to come from the political part of the story, with Billy Campbell (above right) as Darren Richmond, running for mayor of Seattle. That plot has been prominent all along, too prominent not to matter, yet so far has no real connection to the murder. Yes, Rosie's teacher, Bennet, volunteers for a group the campaign supports, and Richmond is making a great show of loyalty to him, but the teacher was too obvious a suspect from the start. My instinct says it might not be Richmond himself - too obvious again - but I'm focusing my guess in that direction. I've never trusted his girlfriend/campaign manager, for one.