Murder mystery

Old Man Murder

Riddle:

An old man, Dave, lives alone in a flat. Because of his age , he is not able to move comfortably and hence most of the things used to be delivered to his house. On Friday while delivering the mail, the postman feels something suspicious in the flat and try to look inside through the key hole and he saw the bloody body of the old man. Homicide arrived the scene. On the outside of flat, they find two bottles of warm milk, Tuesday newspaper, some unopened mails and some gifts. Homicide waste no time and find the murderer almost immediately. How did they do it, and who was the murderer?
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