Little Mistake By The Murderer!

What was the incident?

The police suspected the newspaper straightaway.

Since there were 3 bottles of milk, it's clear that the old man died 3 days ago i.e. on Tuesday & milkman didn't have idea of it. Hence, he has dropped milk bottles there. 

But the newspaper boy knew that the old man is no more in this world hence he didn't drop newspaper since Wednesday.