They're Always Caught (1938)

An honest mayor sets out to clean up graft and corruption in a big city, but when he sees through a headline-grabbing prosecutor who's only going after the small fry, he's in danger. Enter Dr. John Pritchard, who heads up the city's crime lab. He methodically sifts through evidence, uses a spectrograph, looks for fibers and grains of gunpowder, and finally cracks the case. But does his own ability put him in danger?