Hiding the Sex of a Character

While writing the opening scenes of HEIR APPARENT, I decided it would heightened the degree of mystery, to hide the sex of my killer.  The first two victims include a man and a woman, and two of the most likely murderers are a man and a woman.  And I hate to eliminate one or the other by simply referring to the killer as "he" or "she".

I must confess, however, to finding it somewhat difficult to keep the murderer genderless.  Constantly referring to that person as "the killer" or "the murderer" feels a bit awkward.  And there are some characteristically male and female actions that will tend to bend the reader's thoughts toward either a male or female killer.  I have to be particularly thoughtful about these.

I'm interested to know if any of my readers has read a novel where the sex of the killer is successfully hidden from the reader for a considerably portion of the book -- particularly if there are scenes written from the killer's point of view.  Anyone?  Anyone?