The Man Who Fell Through the Earth by Carolyn Wells

The mystery was a baffling one. I learned from Pennington Wise that he had a pipe dream that Amory Manning had killed Amos Gately.

But, save for the faithful Zizi, he could find no one to share his suspicion. It was too absurd. In the first place, had Manning done the deed, he never would have hung around the scene of the crime as he did, for nearly an hour. I remembered perfectly his demeanor and expression, as I saw him, with Olive Raynor that afternoon. He was deeply concerned, greatly shocked, and most considerate and thoughtful of Olive, but there was no shadow of guilt on his fine, strong face.