Diary of a Criminal Investigator

Lu Chuan picked up the murder weapon and observed carefully: "The fingerprint patterns on this weapon are blurry, with severely spread edges. It seems to be a kind of blurred impression formed after the original fingerprints peeled off."

There are many ways to make fingerprints disappear. Some methods make fingerprints disappear for a short time, while others can make them disappear for a long time.

Whichever method is used, it is sufficient for committing a crime.

Regarding the fingerprints on the murder weapon, Lu Chuan leans towards a theory that they were created after soaking in diluted nitric acid, causing the fingerprints to peel off.

The person must have had some research on fingerprints, at least knowing what concentration of diluted nitric acid can remove fingerprints without harming the fingers.

A simple online search for information would not suffice to achieve this.

Online, one can find methods using high-concentration diluted nitric acid to remove fingerprints, but specifically, what concentration? How to mix and handle it? How long to soak?

These details are not something you can learn from simple online searches.

"Are there any warehouses at the logistics base storing chemical substances or other dangerous goods?"

"Yes, there are."

Wang Chuanmin had already been at the logistics base for a week and was very familiar with each warehouse there.

In fact, the entire logistics base is relatively remote, far from the downtown area. After the crime, it was very likely that the biggest suspect was hiding within the logistics base, so after the incident, the criminal investigation team of Tianzhou City focused mainly on investigating the staff within the logistics base.

Wang Chuanmin himself had almost visited every warehouse in the logistics base.

In this regard, Qin Yong was much luckier than Wang Chuanmin because over the past year, although several major cases had occurred in Haizhou City,

they were basically solved in a very short time, so Qin Yong didn’t have the opportunity or time to visit each suspect site individually.

Wang Chuanmin had no choice. When the subordinates were ineffective, he had to step up himself.

"Is there a laboratory?"

"Yes."

"Captain Wang, I need to use it to conduct an identification analysis on the murder weapon."

"Ok, I’ll arrange it right away."

Lu Chuan’s analysis of the murder weapon was not to re-extract fingerprints from it for identification.

The crime scene investigation team in Tianzhou City is actually very capable. Although their fingerprint collection may not be up to Lu Chuan’s level, there haven’t been any significant errors.

With the fingerprints on the knife handle being so blurred, successfully identifying them would be extremely difficult, and even for Lu Chuan, it would require a lot of time and effort.

Moreover, there’s no guarantee of success.

Therefore, what Lu Chuan intended to do was analyze the residues on the knife handle to see if there were any chemical substances similar to diluted nitric acid remaining.

The murderer used a chemical substance to soak the fingers, causing them to peel and the fingerprints to disappear, which would definitely leave some chemical residues in the skin. After testing, it can be proven if such substances are present.

If this is proven, then a key investigation should be conducted on people within the park who come into contact with chemical substances and check whose fingerprints are no longer present. This person is most likely the guilty party.

The crime scene investigation lab in Tianzhou City is well-equipped, although not as advanced as the current equipment of the Haizhou City Criminal Investigation Team, it is much better than Haizhou City’s previous equipment.

Extracting the residues on the knife handle is straightforward; it can be done by soaking in an extraction solution.

However, this type of extraction is a one-time method, meaning that once Lu Chuan extracts the substances from the knife handle, it will be very difficult for anyone else to extract anything in the future, as the amount of residue is very small, and it is almost gone after one extraction.

As for proving the presence of nitric acid, it is even simpler. Just heat the extracted solution in a test tube.

When nitric acid is heated, it will produce brownish-red gas and turn anhydrous copper sulfate blue.

It’s a simple chemical reaction that even a middle school student can perform.

The experiment quickly concluded, confirming Lu Chuan’s hypothesis.

Logistics warehouse, Tianhong Chemical Material Storage and Transport Company.

"This is the only logistics company specializing in transporting and storing chemical goods in all the warehouses at the North Suburb logistics base."

Outside the park, Wang Chuanmin, Lu Chuan, and others had already arrived.

"Start investigating, especially if there are any workers recently injured on their hands; they should be investigated thoroughly."

Wang Chuanmin hadn’t expected Lu Chuan to provide a lead so quickly.

This company had previously been within the investigation’s scope, but at the time, no anomalies were discovered.

Now, all police forces are concentrated here. If the suspect is indeed here, there’s no chance of escape.

But contrary to their wishes, after questioning all thirty-two people in the company, not a single one had injuries on their hands.

Upon hearing the results, Lu Chuan frowned deeply.

It didn’t seem right.

Using diluted nitric acid to soak fingers would result in severe consequences if the concentration was incorrect.

Diluted nitric acid is a highly volatile chemical substance. It’s not that the more diluted its concentration, the lower its destructive corrosion. On the contrary, under certain critical thresholds, low-concentration diluted nitric acid is more corrosive.

"Keep checking, Captain Wang, I suggest we investigate people who left this company before. After the fingerprints are corroded by diluted nitric acid, it takes about three months for the skin to regenerate."

"Investigate all former employees within the last three months!"

"Ok, I’ll arrange it right now."

The HR department at Tianhong Company quickly provided the information on employees who had resigned within three months.

There are three people in total.

"Our company is relatively small, so there isn’t much turnover. These three left due to family issues."

"Zhang Zicai, Liu Gensheng, Wang Yong."

These are the names of the three who resigned.

"Let’s start with this one, Zhang Zicai."

After reviewing the resumes of the three, Lu Chuan first focused on Zhang Zicai.

Among the three, Liu Gensheng was a driver, driving a transport vehicle; Wang Yong was a logistics manager responsible for cleaning the factory area, essentially a janitor.

Only Zhang Zicai, a warehouse manager, regularly interacted with chemicals.

Furthermore, his educational background was the highest of the three. The other two had only primary school education, while Zhang Zicai graduated from high school.

A high school graduate...

As long as his studies weren’t particularly poor, it wouldn’t be too difficult to learn how to mix diluted nitric acid.

Having a target directs the investigation, so Wang Chuanmin arranged for three teams to conduct follow-up investigations on the three who resigned.

This shows the advantage of having more personnel; Lu Chuan had his reasons for prioritizing the investigation of Zhang Zicai.

But with ample manpower, all three individuals can be thoroughly investigated.

However, more resources were dedicated to the investigation on Zhang Zicai.

After resigning, Liu Gensheng continued to work as a driver at a company outside the park, but now he drives a sedan.

"Driving a transport truck often requires staying up all night, and although the pay for driving a truck is slightly higher, the toll on health is too great as one gets older, so he resigned from that job."