* major spoilers *

The ending has several messages ... i.e., apart from being in line with actual events ...

(1) people always like to assume they "know" ... and in a way or another, they force the impression upon others (e.g., the audience) ... they make it "happen" (e.g., the interrogation, etc) ... it is only at the end that we start wondering if we "assume" too often that we "know" ... 

(2) i also like the fact that it mirrors reality; after all, a lot of answers in life are never known, no matter how much we try ... so why shld this be an exception ...

(3) finally when the inspector looks at the audience through the "drain" ... the director is also suggesting, "hey, any of you could be guilty" ... and/or ... "hey, the murderer is still out there in the audience" ...