The gameplay would consist of the Detective asking each of them questions about the scene, and what they are doing there until they are confident they've solved the crime. At which point they say aloud what they think happened or who the culprit is or whatever. If they are right, the players win. Otherwise, they lose, and police go down the wrong track for a while and the case goes cold.
I figure that while individual cases would have limited replay value, this would be an essentially infinitely expandable game. An expansion would literally consist of an 8.5x11 inch picture, a few role cards with the key exclusive information on them, and a card saying what actually happened so you can determine if they detective got it right. In terms of production, a whole case would fit on two 8.5x11 sheets. Probably too small for an expansion, so expansions would need to have several such cases.
Alternatively, cases could be released individually as part of a subscription service. Say a case a month for $9.99 a month.