A cop was walking past a restaurant when he heard someone scream - "No John, not the gun!" He ran inside and and saw a doctor, a lawyer, a milkman, and a dead body on the floor. He promptly walked over to the milkman and arrested him. He didn't witness the shooting and there was no apparent evidence to prove who shot the person and no one told him who the killer was.
The milkman was the only male. The doctor and lawyer were females, so the cop knew that "John" was the milkman.