those people? How did they gain access to the castle? And how
did they leave it?

The police officers of Rouen, convinced of their own impotence,
solicited the assistance of the Parisian detective force. Mon.
Dudouis, chief of the Surete, sent the best sleuths of the iron
brigade. He himself spent forty-eight hours at the castle, but
met with no success. Then he sent for Ganimard, whose past
services had proved so useful when all else failed.

Ganimard listened, in silence, to the instructions of his
superior; then, shaking his head, he said:

"In my opinion, it is useless to ransack the castle. The solution
of the problem lies elsewhere."

"Where, then?"

"With Arsene Lupin."

"With Arsene Lupin! To support that theory, we must admit his
intervention."

"I do admit it. In fact, I consider it quite certain."

"Come, Ganimard, that is absurd. Arsene Lupin is in prison."

"I grant you that Arsene Lupin is in prison, closely guarded; but
he must have fetters on his feet, manacles on his wrists, and gag
in his mouth before I change my opinion."