We must further assume that when she left the room she locked the door behind her and brought away the key in order to suggest suicide.

When she got downstairs she told Thalassa the truth, and begged him to shield her.

He promised to do so, and when the door of the study was broken open he took an opportunity to drop the key on the floor, in order to suggest the idea that Robert Turold had locked himself in his room before shooting himself, and that the key was jolted out of the lock when the door was burst in.

It was an infernally clever thing to do.

That's the case against the girl, Dawfield.