In reply to Daithi O Murachu:
Zodiac was kind of epic! It spanned a 14 year timescale (regardless of what the lack of Jake Gyllenhaal's ageing might have otherwise indicated!)
I'm not up on mysteries/thrillers where the audience is kept guessing, but there are some GREAT court films
Anatomy of a Murder (classic courtroom drama)
Primal Fear (slightly glossy and ludicrous but very watchable, three great performances)
Inherit the Wind (another classic courtroom drama though not exactly a whodunnit)
Judgement at Nuremberg (same director and lead actor as Inherit the Wind but on a much bigger canvas and with a bigger theme...again not a whodunnit)
Ooh I just remembered a GREAT little thriller from the 80s. Black Widow starring Debra Winger and Theresa Russell.