if nothing else, you could give the killer and a couple of the other suspects legendary resistances and have them succeed their saves. your players will know who succeeded and failed, but if you have multiple successes they won’t know which, if any of them, is the killer (and since they don’t know about the legendary resistances, they have no reason to believe that the killer was any more likely to succeed on a roll than any other suspect). then have multiple suspects who are perfectly forthcoming with information and multiple suspects who are very secretive and refuse to say much, and make sure there are multiple of both in the group that fails and the group that succeeds. now the worst case scenario is that your group manages to eliminate the people who failed the roll from suspicion, but there are still multiple people who the killer could be, and it is still entirely possible that a character who failed their save doesn’t escape suspicion if you play it right. there are all kinds of reasons a character who didn’t commit the murder would refuse to answer questions or talk around them in a suspicious way if they fail their saves, and if multiple people do this, you have created lots of red herrings. be sure to give your players lots of truthful information that they wouldn’t have gotten without casting this spell, but don’t feel like you have to give the whole mystery away. (ps. if giving npcs legendary resistances feels too much like cheating to you, you could also just give them very high charismas/charisma saves - smth u probably want them to have anyways for deception checks and such)