The Murder of Roger Ackroyd by Agatha Christie

Half an hour later saw Poirot, myself, and Inspector Raglan in the train on the way to Liverpool. The inspector was clearly very excited.

"We may get a line on the blackmailing part of the business, if on nothing else," he declared jubilantly. "He's a rough customer, this fellow, by what I heard over the phone. Takes dope, too. We ought to find it easy to get what we want out of him. If there was the shadow of a motive, nothing's more likely than that he killed Mr. Ackroyd. But in that case, why is young Paton keeping out of the way? The whole thing's a muddle-that's what it is. By the way, M. Poirot, you were quite right about those fingerprints. They were Mr. Ackroyd's own. I had rather the same idea myself, but I dismissed it as hardly feasible."