Part 215: (Hint 7)
"Come to think of it, the group of nine innocent people includes all five servants, doesn't it?"
"Only the servants can use master keys. They can't hand them over to someone else. And, since people who aren't culprits themselves can't assist culprits, the master keys cannot be used for crimes."
Since it's been guaranteed that all five servants are innocent, we know that the master keys have nothing to do with the crimes.
"Yes, that's right...! And that makes this even more troublesome, correct? All of these closed room murders! Every one of them must be constructed without any master keys...!"
"No. Turn the chessboard over. This just means that they can all be constructed with tricks that don't involve master keys."
"I see...! First twilight, the closed room of the dining hall. Second twilight, Natsuhi's closed room. Starting with the fifth twilight, the closed room of the guesthouse. It seems we must look closely through those again..."
"Let's read it through once more carefully. All of the servants are innocent. Master keys can't be used. Let's think through it again with that in mind."