Attending ex-girlfriend's wedding, arresting the groom's officer on the spot

“There’s a fresh scratch here,” he said, picking up a small piece of paint with tweezers and putting it into the evidence bag. “The oxide layer on the metal surface has been scraped off, so it must have been recently.” He looked around. “But from this height, it would be almost impossible to climb up from the outside unless you were a professional climber.”

"Keep an eye out for any suspicious fibers or hairs around here." Yang Lin, holding a magnifying glass, carefully examined the cracks in the floor. "The deceased's clothing was intact, but the pockets were turned inside out, indicating that the killer might be looking for something." He suddenly stopped. Under the magnifying glass, a brown hair lay quietly in a crack in the corner of the wall. "Yang Sen, over here!"

Yang Sen quickly brought over an evidence bag and tweezers, carefully picking up the hair. "It matches the color of the hair we found in the ventilation duct before, but we can't tell if it belongs to the same person just by looking at it." He put the hair into the evidence bag, labeled it, and said, "We need to take it back for DNA testing."

The two continued to expand their search area, slowly checking along the edge of the rooftop. Yang Sen's metal detector suddenly beeped. He crouched down and found a metal object in the soil near a drain. "It's a cufflink!" He gently brushed away the dirt with a brush. "It's silver, with intricate patterns on it. It must be quite valuable."

Yang Lin leaned closer and examined the cufflinks carefully: "This style doesn't seem like something an ordinary person would wear. Maybe we can extract fingerprints from it." He took out a fingerprint brush and black powder, and carefully brushed the surface of the cufflinks. After a while, a few blurry fingerprints appeared. "Although they're not clear, they're better than nothing. We can take them back and scan them with a laser; maybe we can restore them."

“We also need to check the ventilation ducts on the roof.” Yang Sen looked up at the intricate network of ventilation ducts. “The murderer might have entered the machine room through the ventilation ducts, or hidden something there.” The two men brought over a ladder, and Yang Sen climbed up, opened the cover of the ventilation duct, and shone a flashlight inside. “The space inside is narrow, but there don’t seem to be any obvious traces.” He took out a miniature camera and inserted it into the duct to record. “Just to be on the safe side, let’s take the videotape back and analyze it slowly.”

After checking every corner of the rooftop, the two returned to the elevator machine room. Yang Lin carefully examined the area around the body again. He noticed the electrical wire clutched in the victim's hand; the break was clean, not like it had been torn. "Yang Sen, look at this wire. The break is too clean; it looks more like it was cut with a tool." He took out an evidence bag and carefully placed the wire inside. "Perhaps the murderer used this wire to create the illusion of an electric shock."

Yang Sen nodded and began inspecting the equipment in the server room. "The control system of the hydraulic unit shows signs of tampering," he said, pointing to the code on the control panel. "These garbled characters don't look like they were left from normal operation; someone deliberately interfered with the system's operation." He removed the memory card from the control panel. "The logs inside should record when the system was tampered with and the operation records."

As darkness fell, the cold wind grew increasingly biting. Yang Lin and Yang Sen finished photographing the last remaining trace and began organizing the collected evidence. "Although we've found quite a bit, we haven't yet found any direct clues pointing to the murderer," Yang Lin said, closing the evidence box. "We need to take these back for detailed experimental analysis, hoping to find a breakthrough from them."

Yang Sen shouldered the investigation equipment and glanced at the dimly lit elevator machine room: "Yes, the murderer was very cautious; there were almost no obvious flaws left at the scene. But as long as he was here, he would definitely leave traces. What we need to do is piece these traces together." The two turned and left the scene. The Haorui Building was still brightly lit in the night, and the secrets hidden in the darkness were waiting to be revealed.

Inside the property management office of Haorui Building, the air conditioner hummed softly, mingling with the rustling sound of Wang Shuai flipping through the register. Zhang Jianguo, the maintenance worker who reported the incident, was huddled in a plastic chair, his hard hat spinning repeatedly on his knees, the metal buckle making a soft, clinking sound. "I swear, when I inspected the machine room at five o'clock yesterday afternoon, there really was nothing there!" His voice trembled with tears, his work clothes cuffs still stained with elevator hydraulic oil.

Wang Shuai pushed the recorder forward, the tip hovering over the notebook: "Master Zhang, think back carefully. After receiving the call about the elevator being trapped on the 18th floor at 10 PM last night, who else went into the elevator machine room?" The wind outside rattled the glass, Zhang Jianguo's Adam's apple bobbed, and he suddenly grabbed Wang Shuai's wrist: "Engineer Li! Li Zhiyuan from the technical department! He said he needed to check the control system, and he took my toolbox and went upstairs!"

This clue invigorated Wang Shuai, and he immediately took auxiliary police officer Xiao Li to the technical department. The constant clatter of keyboards in the open-plan office suddenly fell silent. Li Zhiyuan's workstation was piled high with circuit boards, and complex code flickered on the monitor. "Last night? I did go to deal with the elevator malfunction," he pushed up his glasses, his eyes gleaming behind the lenses, "but I didn't go to the top-floor server room at all!" Wang Shuai noticed brown fibers, the same type found on the rim of the coffee cup on the corner of his desk, matching the hair discovered at the scene.

The process of visiting the entire property management office was like walking through a fog. The cleaning supervisor pulled out the duty roster, tracing the densely packed names with his finger: "We patrol the public areas every two hours, but only the maintenance staff have the key to the rooftop." The security captain pulled up the surveillance footage; in the video, the surveillance camera in the 32nd-floor fire escape during the time the victim was killed was under maintenance. "This is fucking weird!" The captain slammed his fist on the table, making the walkie-talkie bounce. "The new camera was just replaced last week, how could it break down at that exact moment?"

In the staff locker room, Wang Shuai crouched down to inspect everyone's lockers. Inside the damp lockers, the uniforms reeked of sweat and disinfectant. When he opened Li Zhiyuan's locker, a silver cufflink slipped from a hidden compartment—a style exactly matching the one found at the scene. "This is what I lost last month!" Li Zhiyuan's face turned deathly pale. "Someone must have deliberately planted this!"

The search for missing persons has reached a stalemate. The information on nearly a thousand employees in the building's tenant database has been repeatedly checked, and not a single person has been absent for more than 24 hours. Wang Shuai stood at the building's front desk, watching the white-collar workers coming and going through the revolving doors, when he suddenly noticed an anomaly in the security guard's logbook: "At 1:11 AM on November 15th, a delivery driver registered to deliver late-night snacks, but the record shows he left five minutes later."

When they found the delivery driver, he was tidying up the insulated box on the back of his electric scooter. (End of Chapter)