Solve this riddle:

One winter day, tragedy struck. John found his friend, Jack, dead. John called the police, stating that he happened to be walking by Jack's house and decided to stop by. He knocked and rang the bell for a long time, but no one answered the door. Then, through the frosted window, he noticed a light in the room. John breathed on the icy window and saw Jack on the floor. The police arrested John as the prime suspect. Why?

John called the police and stated that he was walking by Jack's house and decided to stop by. However, he mentioned that he knocked and rang the bell for a long time, but no one answered the door. Despite this, John said that he saw a light in the room through the frosted window. The fact that John claimed to have seen a light in the room suggests that he knew Jack was inside, even though he stated that he had no idea about Jack's whereabouts until then. This inconsistency makes John a suspect in Jack's death.