Acting on a anonymous phone call, the police raid a house to arrest a suspected murderer. They don't what he looks like but they know his name is John. Inside they find a carpenter, a lorry driver, a car mechanic, and a fireman playing cards. Without even asking his name, they immediately arrest the fireman. How do they know they've got the right man?
They couldn't possibly know they had arrested the right man with such flimsy circumstantial evidence... unless it was in the name of Homeland Security ;-)