A Lonely Man

Riddle: A lonely old man lived in his house in the suburb of a city. He never left the house for long. It was a midsummer Friday when the mailman walked by and called out for the man. There was no answer. The mailman looked into the window and saw him in a pool of blood. When the policeman arrived, he found Tuesday's newspaper, 2 bottles of warm milk, and 1 bottle of cold milk. The next day, the killer was arrested. How did the police find out who it was so quickly?