Lateral Thinking

[langameringue]

Acting on a tip, off the police raid a flat to arrest a suspected murderer.
They don't know what he looks like but they know his name and that he is inside the flat.
The police burst in and discover a bricklayer, a taxi driver, an electrician and a fireman playing cards.
Without any hesitation or communication they immediately arrest the fireman.

How do they know they have got the suspected murderer?