There was an answering knock.
"That is he!" he exclaimed.
He bent down and called through the keyhole:
"Are you inside, Gerald?"
There was a faint sound. He could not hear the words, but he was convinced that it was Gerald's voice.
Now to open the door. It was heavy and the lock was strong. There seemed no way except to use the key. That key undoubtedly the superintendent had. Just then Nelson Hawk came in sight. He had been afraid the secret room would be found.
"Well, gentlemen," he said, uneasily, "you seem to have gone astray. What brought you here?"
"Give me the key of that door!" said Burdoch, sternly.
"I have no key."
"Give me that key!"