He's supposed to give you one more clue? Odd he hasn't been doing that for me even when I got all the clues, not sure if that's a bug or I'm just not noticing
It would be impossible (outside of lucky guesswork) to solve the mystery without the clue he gives you, because you fill out the grid with everything to figure out who was in which room with which weapon with which motive, but none of them implicate the murderer until you get the lab results saying which weapon actually killed the victim – that's the last clue he gives you.
Note that he doesn't give you the clue until you've found all the other clues.
If you have other clues to find, he should say he's still waiting on the lab results and suggests you go look in the mansion some more.