Part 221: (Hint 13)

"The four cousins and Nanjo. In other words, all of the survivors at this point in time. Thinking about it on normal lines, that would make this an impossible crime."
"However, that's where we can turn the chessboard over, correct?"
"You've got it. This actually gives us a simple answer. This twilight proves that there's a culprit who isn't Doctor Nanjo or one of the cousins."
"Other than these survivors, everyone is supposed to be dead. And yet, it was impossible for the survivors to commit the crime..."
"It's absolute proof that one of the culprits faked their death."
"And, since there exists a culprit who faked his or her death, that's absolute proof that there were multiple culprits. Only a culprit can fake an examination and lie...!"
"The fifth and sixth twilights may seem to be a perfect impossible crime, but this one incident alone tells us so much."
"Impossible crimes are a double-edged sword. They have the power to make your opponents surrender, but they also make it possible for theories to break deep into your lines of defense. Long ago, you would surely have surrendered. How far you've come."
"Heheh. It's all because you've trained me so well."

The pair grinned at each other. They had been to the brink of death and back in their many intellectual games before now. They wouldn't be discouraged by a riddle like this.

"Now let's move on to the crimes of the seventh and eighth twilights. I think we're getting pretty close to the core of things."
"Yes, let's get ourselves ready and go!"