Behind Prison Gates (1939)

Det. Norman Craig assumes the identity of a dead bank robber (killed in a police shoot out), Red Murray, so he can infiltrate the prison and find three other bank robbers that the police are looking for. Things are going fine until Craig's partner is found dead in his cell with a bullet hole in his head. The plot thickens as Craig now tries to find the robbers and the murderer of his partner. He pits the three he suspects against each other hoping their greed and paranoia will cause them to turn on each other and rat the other out.