Trivia: The movie makers really wanted the audience to believe that Michael Rooker was the killer, rather than Leland Orser. So much so that Rooker actually played the killer in almost all the scenes where the killer wears a mask. When you see the eyes of the killer through the mask the eyes are Rooker's.
Trivia: The Director, Phillip Noyce, makes a cameo appearance as the man in the bookstore.
Trivia: In preparation for the role, Denzel Washington met and talked with Christopher Reeve, as well as a real-life police officer who was paralyzed in the line of duty.
Chosen answer: The killer used to be a forensic scientist for law enforcement (where or when is never stated). Rhymes testified in court that evidence had been falsified, leading to wrongful prosecutions, the killer was sent to prison for this, where "every day I was brutalised". The killer blamed Rhymes for this, so set out to prove he was better by giving him a series of clues that he (in theory) couldn't solve. It should also be pointed out that Rhymes never met the killer or knew what he looked like.