Had I not been thus overpowered, Larsan would never have entered Mademoiselle Stangerson's chamber that night, and the attack on her would not have taken place." A groan came from Darzac, who appeared to be unable to control his suffering.
"You can understand," added Rouletabille, "that Larsan would feel himself hampered from the fact that my room was so close to his, and from a suspicion that I would be on the watch that night.

Naturally, he could not for a moment believe that I suspected him! But I might see him leaving his room when he was about to go to Mademoiselle Stangerson.
He waited till I was asleep, and my friend Sainclair was busy trying to rouse me.

Ten minutes after that Mademoiselle was calling out, "Murder!" "How did you come to suspect Larsan ?" asked the President.
"My pure reason pointed to him.

That was why I watched him.

But I did not foresee the drugging.