Crime Takes a Holiday (1938)

Following the murder of a policeman and unable to get proof against the real killer, District attorney Walter Forbes takes a novel approach to get there. He induces an ex-convict to stand trial for the killing and gets the set-up conviction he wanted. His purpose is to make the head of the criminal underworld and killer come out into the open, under the impression that because of the conviction of the ex-convict he is no longer in any danger. He then calls the real killer on the telephone, which he has wired to be broadcast over the radio.