The Invisible Killer (1939)

Reporter Sue Walker has too much inside information on the local gambling rackets to suit her sweetheart, Detective Lieutenant Jerry Brown, chief of the police Homicide Squad. When the call comes in that there has been a killing at Lefty Ross' place, a notorious gambling joint, Jerry is peeved when Sue beats him there. He discovers that gambler Jimmy Clark was killed as he answered a telephone call, and his body is riddled with bullets but Jerry can't find any weapon. Sue is amazed to see Gloria Cunningham there. Gloria's father is one of the town's leading reformers and she is engaged to District Attorney Richard Sutton. Ross decides to give Sutton all the information he needs and makes an appointment to go to Sutton's home. Once there, Ross is called to the telephone before he can give any information, and is killed in the same mysterious manner as Jimmy Clark.