Having had a murder occur in the near viscinity of the Adventurers not so long ago, I can attest that it's tricky to make it much of a mystery. There are just so many ways for the characters to get information on who did it, and almost impossible for a perpetrator to block or avoid all of them.
My considered reaction to this is not to worry about it too much. So the players have access to cool magic, isn't that a good thing? It's nice to play a character that can do fun and usefull stuff.
If you do want to make their life hard, I think the best way is probably social engineering rather than rules interference or magical chicanery. Don't have the PCs as direct witnesses, deny them access to the murder site and related artifacts (murder weapon, corpse, etc). That way their problems are to do with dealing with people to get access, rather than using their neat powers.
Simon Hibbs
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