The police may conduct a brief investigatory stop on a person if they have an objectively reasonable belief that the person was involved in a crime. If the witness/victim can't identify anyone, then it would be difficult for the police to claim that they have a reasonable basis to detain a person, who otherwise has a right to be free. Ultimately the State will be required to prove the guilt of the defendant by presenting witnesses in court, so if there are no witnesses then the State won't be able to prove the defendant's guilt and the charges would be dismissed.
Answered on Jan 11th, 2013 at 1:46 PM