Truth, the police have asked for people to come forward. I don't think the law requires them to come forward. If they*are suspected of a crime they could be arrested. When I said "they have to be sure before they pull anyone" I meant that the police could only arrest people if they had a reasonable suspicion a crime had taken place. That's how I understand it anyway. So if the people don't come forward voluntarily (and they may have their own reasons) and nobody recognises them, the police could reach a dead end eventually in their line of enquiry.