The Clue by Carolyn Wells

Next morning Rob went over to the Van Norman house with a clearly developed plan of action. He declared to himself that he would allow no circumstance to shake his faith in his friend, that he would hold Carleton innocent of all wrongdoing in the affair, and that he would put all his ingenuity and cleverness to work to discover the criminal or any clue that might lead to such a discovery.

Although some questions he had wished to ask Cicely Dupuy were yet unanswered, Fessenden had discovered several important facts, and, after being admitted to the house, he looked about him for a quiet spot to sit down and tabulate them in black and white. The florist's men were still in the drawing-room, so he went into the library. Here he found only Mrs. Markham and Miss Morton, who were apparently discussing a question on which they held opposite opinions.