Find the Witness (1937)

Rudolph Mordini (Henry Mollison) is a Houdini-type magician who claims he couldn't have possibly murdered his wife, as he was enclosed, at the time of the killing, in a sealed locked-on-the-outside box at the bottom of the ocean. Cub newspaper reporter Larry McGill (Charles Quigley) scoops other reporters and the police by solving the crime himself.