Crime scene: 162 mystery solving puzzles to find out who the murderer is

Chapter 28 Who Should Be Detained
One winter evening, Jack was strolling on the street when he heard a gunshot and saw an old man not far away falling towards the door and slowly falling down.Jack and the only two people on the street ran over one after another and found that the old man had been shot in the back and was dead.Seeing that both were wearing gloves, Jack asked what they were doing.

The first person said: "I just turned the corner of the street, looked this way, and saw the old man was about to lock the door, when the gun was fired, he fell down in response, and I ran right away."

The second said: "I heard gunshots and didn't know what happened. I saw you two running here, so I followed." The key was still inserted in the lockhole of the door, Jack opened the door, walked in room, call the police.

After the police officers arrived, Jack pointed to a person and said, "Detain him for questioning."

I ask: Who should be detained?Why?
[Answer] First, the action of opening the door is the same as locking the door. If he only sees the old man falling down, he should not know that he wants to lock the door.

(End of this chapter)