Some minor spoilers ahead!
If David Fincher's latest film 'Zodiac' would've been a purely fictional story it probably wouldn't have made it past script stage for lack of closure, a dragging second half and its many improbable events. Which is not to say 'Zodiac' is a bad film, quite the opposite, but the "based on actual case files" story of the unsolved murders by the Zodiac Killer taunts most of Hollywood's drama laws.
The serial killer who called himself the Zodiac and haunted Northern California in the late '60s with a number of random killings has never been caught or identified (technically the case is still open). Part of his notoriety comes from the series of letters the Zodiac sent to the press until 1974, signed with a crosshair symbol and containing threats and ciphers, some of which still haven't been solved.
Even from this brief summary of events, it's already clear this story has no real end, no closure. Partly 'Zodiac', based on the book by Robert Graysmith, circumvents this problem by offering a particular (never proven) theory about the Zodiac's identity. It also introduces two journalists (Jake Gyllenhaal and Robert Downey Jr.) to provide an angle to the story, which is otherwise peopled by many different police investigators (the murders were all committed in different jurisdictions). In hindsight, all these separate, non-cooperative investigations seem improbably amateuristic and bureaucratic.
The first half of the film, roughly spanning 1968 to '74, manages to strike a scary balance between the Zodiac's murders and media threats, and the police and journalists' frantic investigation. It is the standard pattern in the serial killer genre, though deftly executed in a very realistic style - not anything like the gothic atmosphere of 'Se7en' but understated, matter-of-factly scary.
Paradoxically, the second half of the film, from '74 to '91 (!), is much more interesting because nothing much happens anymore. The Zodiac is silent and without any new leads the investigations grow cold, the case is filed and archived. Everyone loses interest, except for one journalist (Graysmith), who becomes obsessed with the case, with solving the puzzle. As he is now mostly analyzing old case files - and is therefore once removed from the actual, previously conducted investigation - his search acquires an increasingly abstract quality, reminding of Auster's 'The New York Trilogy' or 'Spoorloos' (the Dutch original of 'The Vanishing').
In the end, 'Zodiac' is kind to the journalist in providing him with a personal solution to the case. But this 'closure' is shaky enough to leave the viewer with an aftertaste of frustration which is probably the most realistic aspect of the whole film.