Like real life the film didn't have much of a message or shape. Well, some kind of retrospective shape is imposed by a few unconvincing comments at the end, reflecting I imagine the book which underlies the film. This might be a clumsy attempt to conform to the need to tie the ends up, or it might be an ironic sending-up of that imperative. Possibly.
The film has several scenes which in isolation conform to a typical detective story - the interrogation, 'dont go down into the basement', more than one character having a revelatory insight, the cops eating doughnuts. But none of it ever goes anywhere much. The bits don't slot together into a structured murder mystery, not even the kind where the killer gets away with it.
To sum up, well made, baggy, possibly deliberately undermining your desire for a satisfying meaning and conclusion, but not avant-garde or anti-story like David Lynch or anything. Good performances by Mark Ruffalo, Jake Gyllenhaal and Robert Downey jnr. I'm not entirely sure I got what this was all about.