Lateral thinking

The fireman

Acting on an anonymous phone call, the police raid a house to arrest a suspected murderer. They don’t know what he looks like but they know his name is John and that he is inside the house. The police burst in on a carpenter, a lorry driver, a mechanic and a fireman all playing cards. Without hesitation or communication of any kind, they immediately arrest the fireman. How do they know they’ve got their man?
The first person to email the correct answer to Mr Goacher gets 3 housepoints.
Well done to Michael Carter in 7W for being the first to get the right answer: The fireman was the only man. The carpenter, lorry driver and mechanic were all women!