The next day, Wang Mengqi was summoned to the bureau. Chen Shi and Lin Dongxue were responsible for questioning him. Chen Shi said, "Mr. Wang, we meet again."

"If you have something to say, then hurry up and say it. I still have work to do." Wang Mengqi looked impatient.

Chen Shi turned on a laptop and played a video. This was a copy of the surveillance footage from a supermarket opposite Wang Mengqi’s company. Wang Mengqi raised his eyebrows slightly while looking at the video, because the person who left the company in the video was himself. And the time was 4:00 pm on November 19th.

"We asked about your alibi that day. You said that you were talking to Geng Qing using the company's landline, but we investigated the surveillance outside your company and found that you had left at 4:00 in the afternoon that day. Where did you go?"

Chen Shi continued to play the next video, which was surveillance footage from outside the park, then he paused the video. He pointed to a man dressed in black who walked into the park. "I asked you if you have this outfit, right?"

"I have replied already. Those clothes are very ordinary. I’m not the only one who owns them. This can’t be used as evidence.”

"But we found your ride-hailing record for that afternoon at the Internet ride-hailing company. The destination was the park. How do you explain that?"

Wang Mengqi gritted his teeth and knitted his eyebrows.

"Are you thinking that you let one detail slip out of a hundred secrets?" Chen Shi demanded.

"Even if you call for a ride, you don’t necessarily have to get in!" He tried to quibble.

"What beautiful sophistry! It's like when US President Clinton said that the semen found on the secretary's skirt didn’t mean that there was sex involved. Should we ask that driver to identify you?"

"I'm very busy at work, and sometimes my superiors have to cancel the previous plan with a single call, so I may not take the ride even after calling for one." Wang Mengqi explained forcefully while covered in cold sweat.

"We will verify this matter. Regarding your alibi, I think it was forged like this. The call between you and Geng Qing started at 4:00. You attached a mobile phone to the handset of the landline and used it from a distance. In fact, when you presented this alibi, I was a little skeptical. Why did you use your company’s landline to make a call? We investigated your record of using the office landline and found that you only used it to make internal calls and never made outbound calls. Doesn’t it imply something like ‘There aren’t 300 taels of silver buried here.’?"[1]

Wang Mengqi seemed to be infuriated, and raised his chin. "If you have something to say, get to the point. Don't talk in such a roundabout manner."

"We suspect that you killed Geng Zhangle."

"Haha! Just because of this ‘evidence’? The park is a public area that anyone can go to, and I am the murderer because I’ve been to the park? What's my motivation?"

Lin Dongxue handed him a paternity test result. When Wang Mengqi saw it, there was definitely something wrong with his facial expression. Chen Shi said, "Geng Zhangle is your biological father. I think he’s already told you this secret. He arranged for his daughter and Director Wei’s son to have a blind date. He asked you for money to send Geng Qing to study abroad. The purpose was to separate the two of you. Geng Zhangle’s feelings for you were very complicated. You are his own flesh and blood. When he was young, he loved your mother and blood is thicker than water. He obviously liked you, but he definitely could not let you two siblings get married!"

"He liked me?" Wang Mengqi roared. "This old bastard knew who I was from the beginning. He kept asking me for this and that. Later, he asked me directly for money, as if it was his right. As a biological father, he never did anything for me, but now he was there obstructing my relationship with Geng Qing. One day, he told me the truth. I am his son, and Geng Qing and I are brother and sister. Everything was a sin he committed when he was young. Now, we had to pay the debt!"

"So you killed him?"

"No!!!" Wang Mengqi flatly denied. His expression had clearly already betrayed him. "I haven't killed anyone, and I don't know how Geng Zhangle died. Besides, he is my father. How could I possibly kill my father?"

"Geng Zhangle died of a kind of poison. We have already investigated and found that it costs 400,000 yuan to buy this poison. You borrowed 400,000 cash from a loan company, handed it to Geng Zhangle, and then at noon before his death, gave the card to the person who sold you the drug in exchange. That afternoon, you carried out the murder."

After Chen Shi exposed the truth, Wang Mengqi was sweating like rain, but his face still looked stubborn. He laughed, "These are all your speculations. There’s no such thing at all. You said he was killed by poison. What poison is it? If you can't find that out, then it’s sudden death!"

Chen Shi had expected that he would reverse checkmate them like this. After all, this was an unknown poison that he bought for 400,000 yuan. Indeed, even if he was sent to court, he would be released because of insufficient evidence.

This 400,000 was well worth it!

Lin Dongxue secretly exchanged glances with Chen Shi. At this point in the interrogation, it had basically been proved that Wang Mengqi was the suspect, but there was a huge obstacle in front of them.

"Should we detain him?" Lin Dongxue asked.

"What's the point of detaining him?" Today's goal has been achieved.” Chen Shi said, "Let's release him first!"

When leaving the interrogation room, Wang Mengqi's clothes were already soaked in cold sweat. Chen Shi said to him, "Mr. Wang, you and I both know what you did. There will be evidence sooner or later."

Wang Mengqi's arrogance was aroused. Staring at Chen Shi's eyes, he whispered, "Go ahead and investigate. No one can stop us from being together!"

After he left, Lin Dongxue said, "I had expected that it wouldn’t be so easy to get him to confess, but the clues are clear now. We’ll concentrate all our energy into investigating him! I will go and report this to Captain Lin."

"I'll go to Old Peng’s side to see what progress he has made."

Chen Shi went to the forensics department, and was told that Peng Sijue wasn’t there but in the reading room. Chen Shi was able to find Peng Sijue in the reading room. There were several thick books on toxicology, chemistry, and radiology in front of him.

Seeing Peng Sijue's dark eye circles and increasingly pale face, he probably hadn’t slept all night. Chen Shi sat down beside him and asked, "Do you have to work so hard?"

"I might have figured it out..." Peng Sijue's voice was a little hoarse. He rubbed the bridge of his nose. "Accompany me to smoke a cigarette outside."

Out in the corridor, Peng Sijue put a cigarette in his mouth with shaking hands. Chen Shi lit it up for him and said with a pained heart, "You spent the night in the library looking up materials? How young do you think you are? Still staying up all night like this?"

Exhaling a puff of smoke, Peng Sijue mustered up some energy with difficulty. He said, "If I don't figure it out, I can't sleep well at night. Speaking of sleeping, I just took a nap in the library. I got a revelation in my dream that the poison may be potassium!"

"Potassium?"

"This is a radioactive substance, but it’s also a trace element required by the human body. Bananas contain potassium. Excessive potassium can cause arrhythmia and respiratory arrest. If it exceeds the limit one hundred or one thousand times, the heart will stop beating instantly. In both cases of death, it was found that the blood potassium of the deceased was excessive. The first thing I thought of was snake venom, because snake venom would produce this pathology, but I didn’t find snake venom after several instances of spectrum analysis and deproteinization. I had a revelation in the dream just now that it was potassium that killed them, high-dose, concentrated potassium. Potassium is a substance that exists in the body itself. Furthermore, the killer set up an interference, so we couldn’t detect it.”

"Too amazing. No wonder you were a brilliant forensics student. You came up with the answer anyway. The suspect has just left. I'll go chase him down now and formalize the arrest!"

"It's useless." Peng Sijue shook his head tiredly. "This is just a speculation. Although it’s a speculation that’s infinitely close to the truth, it’s still a speculation. We can't prove how the potassium had entered the deceased's body, unless we can get a sample of this poison that proves the business relationship between the suspect and the drug maker!"

"So, we still have to catch Xu Fa?" Chen Shi pondered. The truth was close at hand but they couldn’t get to it in a straightforward manner. This feeling was very uncomfortable!

1. It means to make something obvious while trying to hide something. Origin of the story: A man named Zhang Shan saved up 300 taels of silver after years of hard work. He didn’t know where to store the money safely and decided to dig a hole behind his house and bury the silver taels in it. Then he wrote “There aren’t 300 taels of silver buried here” and left his silver there. His neighbor stumbled across the note and thought it was obvious that silver was buried there. He stole the silver and wrote a note saying “Your neighbor didn’t steal this.” in turn.