Criminal Investigator is a classic thriller in every sense of the word. Dauntless gumshoe Bob Martin (Robert Lowery) is researching the mysterious events surrounding the killing of Joyce Greeley (Vivian Wilcox), a stunning showgirl, when he stumbles upon a clue from a seemingly unrelated case. Martin must communicate with Black (Lawrence Creighton), an inmate that he locked up. As if getting information from a prisoner wasn't hard enough, the inmate happens to be deaf and mute. The information from Black, combined with some clues from Greeley's past, uncovers information that leads to a pair of suspects in Miss Drake (Jan Wiley) and Soapy (Charles Hall), but Martin is able to quickly surmise that they are merely pawns. There is someone powerful above Drake and Soapy calling all the shots, and Martin has to find out who that is, to prevent any future murders. Directed by Jean Yarbrough, this 1942 crime drama was released by Monogram Pictures.