"By the way, there's one more thing. When Old Ding sleepwalks, he not only reads books but also practices calligraphy. When we go back later, we might just catch him in the act. If we compare his handwriting with the note left at the scene, it will be clear whether he's the killer or not," Xu Mo added.
The officers, lacking knowledge in psychology, found Xu Mo's explanation hard to accept at first. After all, they had only seen cases of dissociative identity disorder in TV dramas and had never encountered such a person in real life. It was difficult for them to accept this immediately.
At half past ten, the officers drove to a spot just over a hundred meters away from the mental hospital and stopped. They got out of the car and decided to proceed on foot, sneaking closer to the hospital. Before the operation, Xu Mo repeatedly reminded them to move as quietly as possible, not to alert Old Ding in the guard room, and not to give him any time or opportunity to react.
The well-trained officers crouched low, quietly bypassed the main gate, and arrived at the door of the guard room. At Xu Mo's command, they kicked open the door without hesitation and stormed inside.
At that moment, Old Ding was sitting at his desk, holding a pen and writing something. Beside him lay a copy of *Dream of the Red Chamber*. The sudden intrusion startled him.
When he saw the officers returning, a flicker of panic flashed in his eyes. He quickly hid the notebook on the table, glanced around, and, finding no place to conceal it, clutched it tightly to his chest. He took a step back and picked up an old aluminum flashlight from the table, as if preparing to confront the officers.
These were all instinctive reactions.
After a moment of hesitation, Old Ding's expression returned to normal. Realizing how strange his behavior must have seemed, he put down the flashlight and forced a smile, trying to appear as calm as possible.
"You... why are you back? You scared me... I thought it was someone else."
"Hand over the notebook," Xu Mo said, almost certain of his suspicions now. He saw no need to waste time with small talk.
Old Ding hid the notebook behind his back, unwilling to surrender it.
"I was just doodling. There's nothing worth seeing."
Seeing this, Xu Mo stepped forward, ready to snatch the notebook.
Old Ding, the guard, was no match for Xu Mo, who was skilled in martial arts. In just one move, Xu Mo had him pinned to the desk with one hand.
With his right hand restraining Old Ding, Xu Mo reached out with his left and took the notebook Old Ding had been protecting. He placed it on the table and prepared to open it.
"Don't open it!" Old Ding struggled violently.
He knew that once the notebook was opened, it would all be over.
But no matter how much he struggled, it was futile. Xu Mo's grip was like an iron clamp, holding him firmly in place.
Han Fei and the other officers also stepped forward, glancing at the notebook Xu Mo had just taken from Old Ding.
As they opened it, the contents of Old Ding's writing came into view.
After just one look at the handwriting, Han Fei and the officers couldn't look away.
The officer in charge of evidence retrieved the note left at the crime scene, which was stored in a transparent, sealed plastic bag.
This officer placed the bag on the table next to the notebook.
"Captain Han, Captain Xu, the handwriting is identical!"
The handwriting in the notebook was exactly the same as that on the note left by the killer at the crime scene.
Xu Mo's guess was correct. Old Ding was indeed the killer.
Or, to be more precise, the killer was Old Ding's alternate personality.
After a moment of shock, Han Fei glanced at Xu Mo and then turned to Old Ding, who was still pinned down by Xu Mo.
"Do you have anything to say for yourself?"
Old Ding, who had been struggling earlier, fell silent when he saw the notebook opened and the officers comparing the handwriting with the note.
At this point, there was no use in struggling anymore.
After a moment of silence, Old Ding looked at Xu Mo.
"How did you figure out it was me?"
Even though he had been caught, Old Ding was still puzzled. Only he knew about the murders—his primary personality was unaware.
The primary personality had no knowledge of the killings, so when questioned by the police, he could act completely naturally. After all, he genuinely didn't know he was the killer. Logically, the police should never have suspected him.
"I started to suspect you when I saw the copy of *Dream of the Red Chamber* by your bedside," Xu Mo replied honestly. "And when I found out that you sleepwalk and write at night, my suspicions grew stronger."
At this point, Xu Mo picked up the flashlight from the table and pointed the dented end toward Old Ding.
"And this—the shape of the wound on the victim's head matches the end of this flashlight. Your flashlight has a similar dent, which means it was the weapon you used, right?"
"You have dissociative identity disorder. The primary personality doesn't know about the alternate personality, but the alternate personality knows about the primary one. The Old Ding we saw during the day wasn't the killer, but you are. He doesn't know you exist, so he acted completely natural during questioning. If I hadn't noticed other details later, I might have completely dismissed my suspicions of you."
Xu Mo was well-versed in psychology, and its application wasn't just limited to detecting lies.
Psychologists also fall under the umbrella of psychology.
Xu Mo knew the symptoms of dissociative identity disorder, and he knew that the primary personality of someone with the condition wouldn't be aware of the alternate personality. That's why the primary personality had no idea he was the killer and acted so normally.
Another point was that the killer was unusually brazen—so much so that it felt like the killer was mentally ill, which was why he dared to taunt the police like this.
If Old Ding also had a mental illness, such as dissociative identity disorder, and his alternate personality was prone to extreme behavior, then everything made sense.
Hearing this, Old Ding looked both shocked and pleased.
"You know I'm not him?!"
Xu Mo not only knew he was the killer but also recognized him as Old Ding's alternate personality!
Although Old Ding had dissociative identity disorder, he only liked to read books at night and didn't engage in any other unusual behavior. As a result, the staff at the mental hospital hadn't realized that their guard was also a patient. The staff simply thought Old Ding was sleepwalking.
In reality, if they had observed him more closely, they would have noticed that his symptoms weren't sleepwalking but rather dissociative identity disorder. But they hadn't paid close attention to Old Ding or given it much thought. After all, who would focus their energy on observing a guard with a supposed sleepwalking disorder?