Guess the Column Solves the Case

The murder inside the elevator
After recovering from a car accident, the famous painter Wang Yong can only get around by wheelchair. Although his paintings are valuable, he never sells them and only gives them to friends or charities. Wang Yong's residence is a five-story detached house. For convenience, he installed a dedicated elevator. Coincidentally, his younger brother Wang Yuan recently lost his job, so Wang Yong asked him to be an assistant who could also take care of his daily needs. The two brothers got along well. One day, Wang Yong's classmate Lin Fang came to visit him. Lin Fang also used a wheelchair, and this time he brought Mr. Zhu from the charity organization to discuss with Wang Yong whether they could donate to a hospital. When Lin Fang and Mr. Zhu entered, Wang Yuan took the initiative to greet them. After inviting them to sit in the lobby downstairs, Wang Yuan used the walkie-talkie to call Wang Yong upstairs, asking to take the guest up to the fifth-floor studio, but Wang Yong insisted on going downstairs to meet the client. At that moment, we saw the elevator stop briefly on the fourth floor, then came down. As soon as the elevator reached the bottom, the automatic doors opened. We saw Wang Yong dead inside the narrow elevator; His neck was pierced by a sharp dagger, and a thick rubber rope was tied to the hilt of the sword. Wang Yuan walked into the elevator, pushed Wang Yong's body and wheelchair out together, and checked his pulse. His pulse had stopped. "Strange, could there be someone else in the studio on the fourth floor?" "Besides the elevator, are there any other Taiping Stairs?" Lin Fang and Mr. Zhu asked Wang Yuan. "Yes, there's also an emergency spiral staircase. If the killer is really upstairs, arresting him would be like reaching into a bag." "So let's split into two groups to conduct the search." Lin Fang, in a wheelchair, took the elevator up. Lin Fang reached the fourth floor, but didn't see a single person. He glanced at Wang Yong's studio, where the drawings were scattered on the floor; At that moment, Wang Yuan came up from the spiral staircase, panting heavily. Mr. Zhu used the studio's phone to notify the police, then followed Wang Yuan into the elevator's vertical shaft. After a while, he was the only one who crawled out, his hands, feet, and pants covered in dust.
The windows of the fourth-floor studio at the scene were all fitted with iron bars, so the killer had no way to escape through the windows. Wang Yong was killed while taking the elevator downstairs. The elevator never stopped from the fourth floor to the first, so the killer couldn't escape the sight of the three people.\At this moment, Lin Fang suddenly remembered that when he took the elevator earlier, he saw an air vent on the top panel of the elevator.\ “Oh, so that's how it is. I'm sure the murderer is definitely his younger brother, Wang Yuan. Before we arrived, he had already set things up. When the police come later, you just capture him and hand him over to the authorities.”\ Do you know why that is?