Depends on a lot of things really.
First: How many police were there? If there is only one, examine carefully if the police is a stickler for rules or not, and if not try bribing him
Second: Ask yourself if you have left any fingerprints in the knife. If you are sure there aren't any. Feign innocence. It's your word against theirs.
Third: Again, how many police were there? Were you somehow caught by a CCTV camera? If not, feign that the police planted it all up. Again, It's your word against theirs.
Of course I highly discourage even trying this three and just simply apologize and own up to your mistakes.