Solve-it

What clue convinced Detective Ball that this was a murder case?

The index finger print on the trigger.

Anyone shooting himself in the face with a shotgun would naturally push against the trigger with the thumb rather than with the index finger. This death was therefore a case of murder.

Keenan said he heard the gunshot but never saw the murderer. Since there was nowhere to hide in the cabin (sparse furniture, small bed, no bathroom or closet) and no back door or window, this would have been impossible. Therefore, he must have been the murderer.

The first eighteen chapters of Katz's manuscript were missing. This confirmed the murder and indicated a motive. Keenan, desperate for literary success, had called on his neighbor, killed him, set up a suicide scenario, and taken the manuscript (leaving one chapter at the scene to place Katz at the typewriter) with the intention of completing and publishing it himself.