Extracting DNA information substances from blood is somewhat simpler than extracting DNA information substances from other carriers like bone, hair, or skin flakes.

Most of the substance in blood is water, and extracting genetic material involves stripping away other impurities, a process known as the purification of genetic material.

The purification of DNA substances varies slightly depending on the carrier.

Purifying genetic material from blood is the simplest, whereas extracting it from bone and other carriers requires slightly more complex purification substances.

The purification substances used by the Criminal Investigation Team are standardized compositions. 𝒇𝙧𝙚𝓮𝙬𝙚𝓫𝒏𝓸𝓿𝓮𝒍.𝓬𝙤𝓶

The type of purification material to use for different carriers is pre-defined.

Unless under extremely special circumstances, crime scene investigators are not allowed to mix their own purification substances for DNA extraction.

Lu Chuan was taught this by his professor when he first encountered DNA extraction during college.

The fundamental reason is the issue of legality.

Crime scene investigation is a field with strict principles.

Whether the conclusions drawn from experiments by crime scene investigators can become judicial evidence heavily depends on the legality of the evidence extraction process.

The legality of the process includes both the process of acquiring evidence and the handling of the evidence.

Crime scene evidence handling follows specific normative requirements; all behaviors must meet these requirements to become judicial evidence.

Yang Sen and others, with more than ten or twenty years of experience, are well-versed in these procedural norms.

"DNA information from three people has been isolated."

DNA material from three individuals has been isolated from the blood stains wiped by the perpetrator, and Yang Sen is continuing the work.

If the perpetrator left their own blood at the scene, theoretically, DNA information from five people can be isolated from the blood stains wiped by the perpetrator.

Four of these should belong to the four victims, and the other one should be the perpetrator’s.

Yang Sen is working on isolating the DNA from the wiped bloodstain, while Old Bai and Liu Guodong are extracting the DNA information from the other four people.

Afterward, comparing both sides will yield the perpetrator’s DNA data.

Lu Chuan is also busy; the perpetrator left footprints at the scene, and Lu Chuan is doing the footprint identification and analysis report.

Although Lu Chuan has roughly judged the perpetrator’s body type characteristics on the scene, issuing an analysis report is different from an oral statement.

The footprint identification report must have judicial validity, so data analysis of the footprints is quite important.

This is not something that others will believe just because Lu Chuan says so; it needs sufficient data support.

Moreover, there is the fingerprint extraction at the scene.

The crime scene was in disarray, with many places having residual fingerprints, and the perpetrator did not clean up the fingerprints after the crime.

Wiping fingerprints was actually the most time-consuming task for the four people at the scene.

Based on the blood traces at the scene, Lu Chuan roughly reconstructed the crime scene. The perpetrator’s speed was probably fast, and they didn’t linger on the scene, took the murder weapon, and a pillowcase from the bed was taken away by them.

Theoretically, objects that might have residual fingerprints are gone; in other places, the likelihood of the perpetrator leaving fingerprints is actually not high.

Lu Chuan is currently focusing on the fingerprint identification extracted from the door.

Estimating from the victim’s time of death, the perpetrator committed the crime between 3:00 a.m. and 4:00 a.m. today.

Being able to attack four sleeping victims indicates that the perpetrator must have had a key to the room.

Thus, if the perpetrator did not wear gloves when opening the door, they likely left fingerprints.

The action group responsible for the investigation also hypothesizes that it was committed by someone familiar with the victims.

The crime scene environment is quite complex; the entire building where the landlord rents has victims. The perpetrator finding their way accurately to the victim’s home shows they are very familiar with the building’s situation.

This is also one of the key reasons why Wang Junshan is included as an important suspect. Last year, he lived with Zhang Xiaotian, and he must be very familiar with the environment here.

Moreover, since they lived together, having a key to the room wouldn’t be surprising.

While the crime scene investigation proceeds, Wang Changjiang and Shen Yan also begin the autopsy on the bodies.

The cause of death is relatively easy to determine. All four victims died from the same sharp weapon, with cuts to the neck and stabs to the chest and abdomen causing massive blood loss leading to death.

"There are signs of a struggle at the scene, but why didn’t the neighbors hear calls for help?"

Shen Yan is somewhat puzzled.

From the traces at the scene, the two male victims might have lost strength quickly due to damage to the carotid artery, but what about the other two women?

The two female victims had clear signs of struggling with the perpetrator. Why didn’t they call for help during the struggle?

The soundproofing of the rental room wasn’t very good, so if the two female victims had called for help loudly, those nearby should have been able to hear.

Yet the reality was that no one called the police during the process, and no one discovered the family was victimized from the investigation results.

"Take them back for an autopsy."

Shen Yan, not being a frontline investigator, although she has questions, doesn’t delve deeper as conducting autopsies is the forensic pathologist’s job.

Although there are four bodies on the scene, the autopsy work is relatively straightforward. The cause of death is clear, and the time of death is also relatively clear. The focus for the forensic doctor is mainly on toxicological analysis to see if there are any other special circumstances.

Shen Yan overlooks something because she is a forensic doctor, but Zhang Hui and others from the frontline criminal investigation won’t overlook such obvious doubts.

On the way to arrest Wang Junshan, Zhang Hui and others discuss the case.

"Brother Hui, the investigation situation in the building... does seem a bit off."

"Hmm? What aspect are you referring to?"

"Based on the time of death, the victims were killed around 3:00 a.m. to 4:00 a.m. today."

"This time is less than an hour from dawn, and some of the residents in the building work at the vegetable market selling produce."

"Logically, they should wake up early. How come they didn’t hear any commotion?"

"Moreover, there are signs of struggle between the two female victims and the perpetrator. The two female victims weren’t killed instantly, so why didn’t they call for help during the fight?"

"The soundproofing of the rental room... When we were investigating next door to the crime scene, we could hear Liu Guodong and others investigating next door, the soundproofing is so poor, and yet the neighbors didn’t notice the sounds of a struggle?"

"It doesn’t make sense."

Zhang Hui nods. There are a total of seven households in the building, one of which is right next to the crime scene, while the other six are either upstairs or downstairs, but not far away.

Even though 3:00-4:00 a.m. is a time when people are deeply asleep, like Old Zhao said, seven households didn’t hear a single sound?

Why?

Two reasons: either there was no noise at the crime scene, or... these seven households are lying.