Acting on a tip off,the police raid a flat to arrest a suspected murderer.They dont 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 immediatley arrest the fireman.
How do they know theyve got the suspected murderer

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(answer to follow soon) :!: :!: :!: :)