I just googled Mind Trap, and that's the game OP is thinking of, we had it, I remember the design of the game box. Mastermind involves colored pegs, I played it while camping with my family a lot.
If you like this style of game, look for one called Crack the Case, too. It sounds exactly like the game you described except they're not all murders, but a variety of crimes. We loved playing it in the car as kids!
Lizi
We just found Mindtrap in my inlaws basement. Would this game be appropriate for elementary ages kids on a road trip? Or is it more for teens and grownups?