Chapter 360: The Fourth Person

Normal human urine is typically colorless and transparent or slightly yellow; although it contains large amounts of metabolic waste, it contains no bacteria, so it can be consumed.

Additionally, normal urine is weakly alkaline, so for those pursuing health and wellness who insist on drinking alkaline water, urine can also be an option.

At the same time, urine provides a great deal of information: hospitals use it for urinalysis, and forensic institutes use it for DNA extraction.

Urine contains small amounts of shed cells, which can be used for DNA matching.

Laypeople see the surface, but experts cannot pretend ignorance.

While DNA can indeed be extracted from urine, the difficulty is far higher than with blood or similar samples. First, the number of shed cells in urine is low, requiring a larger volume for convenient extraction.

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On the other hand, the urine at the scene is clearly poorly preserved; ideal urine for DNA extraction should be stored at low temperatures.

Therefore, during the early adoption of DNA technology—the first decade of the 21st century—extracting DNA from urine was nearly impossible, and similar cases were extremely rare. Back then, DNA matching relied primarily on blood and semen, and secondarily on toothbrushes or cigarette butts containing oral epithelial cells; even hair samples were only useful if they contained follicular tissue.

Today, technology has improved, and the richness of DNA matching has increased dramatically. Hair no longer requires follicular tissue, and top-tier DNA labs now have very low requirements for extracting DNA from urine.

By the way, here's a little-known fact: if a man or woman suspects a child is not theirs, they don't need to take the child to an institution or draw blood to leave a trace—just collect a sample of the child's urine, chill it in an ice-cold soda, and send it to a testing lab; results come quickly.

For suspects, it's even more convenient. Jiang Yuan used a damp cotton swab to collect as much urine stain as possible, then combined several swabs, wrapped them in filter paper, and folded them into a neat triangle.

This method maximizes the integrity and original state of the urine sample, identical to extracting bloodstains.

Several forensic technicians from Chiyong City watched Jiang Yuan's actions, unable to grasp his approach.

Frankly, Jiang Yuan's procedure today had nothing mysterious about it—it was simply the most basic and cautious method, and none of them could say whether it was good or bad.

But the case had clearly made progress, and that was a very real achievement.

"Urgent the DNA lab," Jiang Yuan handed the urine sample to the technician behind him and continued searching the bathroom.

Liu Jinghui interjected: "This person has a 90% chance of being strongly connected to the case—likely a participant or someone with knowledge."

The team leader immediately asked: "Why?"

Liu Jinghui said: "This is the hideout of the three-man gang—essentially where they planned to rob and dispose of stolen goods. Aside from the three and their accomplices, no one else would be brought here."

The team leader nodded repeatedly. He wasn't particularly skilled at solving cases; rather, he had some criminal investigation ability but leaned more toward management and had gradually moved away from frontline work. He didn't trust his own judgment on such matters, but when Liu Jinghui spoke, his mood shifted immediately.

"So the actual criminal must be four people?" The team leader strained his mind to calculate 3 + 1.

Liu Jinghui glanced at Jiang Yuan and smiled: "At the crime scene—the victim's home—the perpetrators were definitely three people. As for the owner of the urine we just found, I speculate… well, speculate, of course—that he's the fence."

The team leader blinked: "A fence? Why?"

The answer was somewhat off-topic, but Liu Jinghui explained slowly: "First, this person doesn't seem like an accomplice. All three suspects are young men in their prime, tough and aggressive; this one runs off to pee in three different directions—what use is such an accomplice? He'd just be taking a cut of the money."

"Second, could he be the mastermind? But from the safe incident, he provided no corresponding tools—like an oxygen cutter. If he were the mastermind, he should have bought or borrowed it. But our investigation clearly shows the three went together to that auto dismantling yard to buy the oxygen cutter, proving they didn't trust each other—or him."

"Third. This man appeared at the gang's hideout because the three needed him; otherwise, they wouldn't have told him the location. Those three young men, fresh off a major crime and preparing to flee, need what most? Money, documents, and an escape plan."

Liu Jinghui lifted his chin: "From our perspective, documents and escape plans are harder to get. But in reality, the most valuable and hardest to obtain is still money. The victim's cash was minimal; the main losses were jewelry, gold, watches, antiques… The three must convert these into cash to flee, so right now, what they need most is a fence."

"Makes sense, makes sense…" The team leader nodded repeatedly. He now saw hope of solving the case, and every word Liu Jinghui spoke sounded perfect to him.

With the conversation over, the room fell silent.

Both Liu Jinghui and the team leader frowned.

This atmosphere was wrong.

During crime scene investigations, people often talk—even chat—is normal.

It's like finding a condom and a hardened tissue—you discuss them to determine whether one or two encounters occurred.

Just now, the forensic technicians following Jiang Yuan had been murmuring quietly, like background noise in a forest. Now, suddenly silent, it was as if the forest had become utterly still—clearly unreasonable.

Liu Jinghui and the team leader moved closer and saw at the bathroom entrance, several birds… no, several forensic technicians, all craning their necks to look at something.

Further inside, they saw Jiang Yuan holding a tablet, gesturing with his fingers.

"What… is he doing?" the team leader asked curiously.

"Separating overlapping fingerprints," the oldest technician turned back, his eyes clearly filled with despair.

The team leader blinked: "Found overlapping fingerprints? How are you separating them?"

"I said," the technician repeated.

Liu Jinghui looked at Jiang Yuan and whispered: "What exactly are we doing here?"

"Found a few fingerprints on the toilet flush button—they overlapped. I want to separate one first and see if it matches the fourth person," Jiang Yuan said simply.

The apartment had many fingerprints, and they were chaotic. Given the three-man gang had rented the place for only a short time, most fingerprints likely came from the landlord and previous tenants, and even earlier ones.

Although all fingerprints could be collected and compared collectively before filtering, Jiang Yuan had other methods and refused to wait.

After all, DNA had already been collected—the most compelling evidence—so collecting fingerprints now was primarily for speed.

In manhunts, an extra hour is an extra hour; you cannot give the killer any breathing room. Only by making the fugitive feel suffocated can the pursuit succeed.

Therefore, Jiang Yuan's choice was simple: he took fingerprints directly from the toilet flush button.

The fourth person urinated in the toilet; normally, he would press the flush button, so the flush button likely bears his fingerprints.

At the same time, the flush button also bears fingerprints from the other three, and possibly more people.

The problem now is that overlapping fingerprints cannot be distinguished or matched by software—yet this is only the understanding of ordinary forensic technicians.

At Jiang Yuan's level, if he wants to, he can separate fingerprints by analyzing their ridge characteristics.

The process isn't easy, but Jiang Yuan can do it.

Yet this sight left ordinary forensic technicians heartbroken—such skill requires innate talent, divine favor; it's nearly impossible to learn otherwise.

Even Jiang Yuan analyzed each ridge one by one.

Modern fingerprint technology theoretically can decompose overlapping fingerprints via software, but the error rate is too high; the results are essentially unusable. A knowledgeable lawyer could easily dismantle them in court.

At this point, manual analysis remains the most reliable—and extremely difficult.

Yet Jiang Yuan accomplished it effortlessly with just a tablet.

After marking two sets of feature points, he quickly matched one among more than ten.

"Re Fuhan. He has a prior record for fencing stolen goods and also deals in fake documents—his services cover everything." Jiang Yuan read the match result, feeling this man was tailor-made for the case.

The team leader slapped his thigh hard: "Did the three-man gang buy their fake IDs from him?"

"Very likely," Jiang Yuan nodded.

"It must be! Fencing and providing documents!" The team leader was already eager.

If the gang's identification documents were bought from the fourth person, capturing Re Fuhan would easily reveal the gang's current location.

Once they arrested him, the entire case would be perfectly resolved.

The team leader grew increasingly delighted, picked up his phone, and began making arrangements.

At this point, Jiang Yuan agreed with his judgment and turned quietly to Mu Zhiyang: "You can buy high-speed train tickets to go home."

……