Information about WTC:

The World Trade Center Seoul (WTC Seoul), commonly known as the COEX, is a building complex centered around the COEX Convention & Exhibition Center on Teheranno in Samseong-dong, Gangnam-gu district of Seoul. It contains the ASEM Tower, the Trade Tower, and the COEX InterContinental Seoul. The 54-story building Trade Tower was built in 1988 with a height of 228 meters (748 feet), and is one of the tallest buildings in South Korea.

The 836-meter (914-yard) section of sidewalk along Yeongdong Boulevard from exit No.5 of Samseong Station on Seoul Subway Line 2, outside Convention & Exhibition Center and ASEM Tower is designated as a smoke-free zone by the Seoul Metropolitan Government.

Its facilities are...

• COEX Convention & Exhibition Center
• Trade Tower
• COEX Mall - underground shopping center and Kimchi Field Museum
• COEX Aquarium
• ASEM Tower
• COEX Artium - opened in April 2009, it is a theatre dedicated to musicals
• COEX InterContinental hotel
• Convention Annex
• Oakwood Premier COEX Center - a luxury serviced residence
• City Airport Tower
• Korea City Air Terminal
• Grand InterContinental Hotel
• Hyundai Department Store

There is an upcoming event in COEX Artium, it is the Legally Blonde musical.
Since the premiere of the musical has been drawing near, Jaeyu has been spending more time in the Artium to clean up.
I decided to talk some of the staff there. My father who is a composer and has previously participated in a musical at the Artium before has some friends there who recognized me. I asked them about Jaeyu and they said he was just a simple janitor who was very friendly to them. One of them told me that he did have a nemesis in WTC though. His nemesis was a young man named Oh Junyoung. Oh Junyoung works at at the Bandi & Luni’s bookstore at COEX Mall. Rumor has it that he once liked Jaeyu's daughter Eunah but Jaeyu never got along well with Junyoung. I proceeded to the bookstore and observed Junyoung. I decided not to ask him yet about the murder because I have to avoid revealing myself to him as a Community Affair representative. A book that Junyoung was reading caught my eye. It was a book about some of the ways of murdering someone. It's possible that he is just interested in things like those or he has a reason why he is reading that. I pretended that I was a niece of Jaeyu and asked him about the fire in WTC and Jaeyu's death. He tried to avoid answering me. He claimed that he did not know that Jaeyu was already dead. While he was attending to some customers, I did a little bit of snooping at the table where he was reading his book. I found two red strings in the book that he was reading. The red strings were quite similar to the one found on Jaeyu's hand. I took one and put it in a plastic bag. I am not sure what he was doing with the strings, maybe he was using them as bookmarks. Anyways, I think that Oh Jaeyu is capable of being our 3rd suspect as he has a big body build that would enable him to beat Jaeyu up and he is familiar with weapons and stuff that you can use to kill a person. I also found this receipt near his notebook which states that he bought hydrogen cyanide. Despite that, I am still not sure if Junyoung really has the motive to kill Jaeyu.

Additional info: (Sungyeol helped me with this.)

• Jaeyu used to be really close with his daughter as seen in the albums in their house.
• Jung is their surname
• The door to his house might have been locked or not. 
• I found out that Jaeyu is divorced with his wife. His wife now works as a barista at a small cafe. According to her, Eunah used to be a very good daughter. Eunah became rebellious because of Jaeyu's overprotective attitude towards her.
• Another eyewitness: Cho Mina. She claims that she always sees Junyoung fighting with Jaeyu. Mina works at the artium as a stage manager.
• Eunah has all of Jaeyu's money now since she is an only child.

Some theories by Lee Sungyeol: 

• "I think that the daughter is definitely involved in the murder. She's his daughter, so she's bound to have a replica of the key. She could have then passed it on to someone."
•  "Maybe... the old man did something bad to the murderer, which made him set off a fire in WTC, hoping to kill the man. But, he didn't succeed, which is why he murdered him at home. Do you think that might be possible?" (However, I think that there might be two or more murderers. This is just a theory. Junyoung didn't cause the fire because that's where he works at but he is still capable of murdering Jaeyu in his house.)