The door resisted her impulse.

It was locked.
She tried it twice, and it was only as she drew back the second time that she saw the key lying at the foot of the door.

That deepened the mystery.

Why had they locked her in?
Why, when they had done so, had they thrust the key under the door and so placed it in her power?
Had Claude Mercier done it that the others might not enter to hear what he had heard and discover what he had discovered?
Possibly.

In which case the knocker--who at that instant made a second and more earnest attack upon the door--must be one of the others, and the sooner she opened the door the less would be the suspicion created.
With an apology trembling on her lips she hastened to open.