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?