"Keiichi-san was startled by the knock and got up and walked near the door. But as he would usually feel ill after waking up, he must've felt drowsy then. As he walked towards the door under this state, he suddenly remembered."
"Remembered what?" Haruhi asked. Koizumi replied to her with a smile,
"He remembered that he was killed by his brother. In a flash, the image of Yutaka-san wielding a knife reappeared in his mind, and Keiichi-san frantically locked the door."
I couldn't stand this any longer, so I cut in,
"Are you saying that's the whole truth behind this sealed room?"
"Unfortunately, that's what I believe. Keiichi-san had lost all sense of time after fainting, he probably thought Yutaka-san had returned to finish him off. I think it was only a few seconds from him locking the door to us holding the door knob."
"If the killer had wanted to return to give him the finishing blow, why would he knock on the door intentionally?"
"Keiichi-san's mind was very blurry then, so this was the swift conclusion he made with his half-conscious mind."