The ONLY time that a law enforcement officer can not lie is A COURT OF LAW.
At all other times it is to be assumed that what they are telling you is a lie.
SIMPLE RULES:
1) PLAY DUMB
2) DO NOT ADMIT TO ANYTHING, just say something like "I'm confused, I don't know what's going on, and I'd better get a lawyer because what you're talking about sounds like a serious crime." And leave it at that.
3) COPS CANT MAKE DEALS TO LIGHTEN YOUR SENTENCE. Only the prosecutor or judge can do that.
4) BEING "TRUTHFUL" TO COPS IS THE STUPIDEST THING YOU CAN DO.
5) TRYING TO CONCOCT A STORY (LYING) TO COPS IS THE SECOND STUPIDEST THING YOU CAN DO. (just don't say ANYTHING!)
6) IF YOU GIVE UP YOUR SUPPLIER, THEN THE COPS HAVE TWO ARRESTS INSTEAD OF ONE. THEY WILL NOT, I REPEAT, NOT LET YOU GO IF YOU GIVE SOMEONE UP. No matter how many times they say that or how many different ways they say it, don't believe it. EVER.