She kept wondering if Chief Fleck had made any further discoveries about the murder and resolved to ask him about it at the first opportunity.

She therefore was delighted when on Tuesday, as she made her regular report by telephone, he asked if she could come to his office that afternoon with Dean to discuss some matters of importance. They found Carter already with the chief when they arrived.

"Thanks to your work, Miss Strong, and to Dean's dictograph," said the chief, "we have made considerable progress. We have learned a lot more about the cipher messages."

"You have learned it through me," cried Jane in amazement.

"Yes," said the chief, smiling, "from that list of names you reported."

"What were they, a cipher, a code?" questioned the girl breathlessly.

"No, nothing like that. They are merely the names of various innocent and unsuspecting booksellers in various parts of the city."

"How did you discover that?"

"In the simplest and easiest way possible. I listed all the names you reported and studied them carefully, trying to find their common denominator. They were not in the same neighborhood, so it was not locality. They were not all German, so it was not racial. I looked them up in the telephone directory, checking up the numbers of the telephones of the Jones, the Simpsons, but that gave no clue. Then, as I looked through the telephone lists, I discovered that there was a bookstore kept by a man of each name. Then I understood. It is a simple plan for throwing off shadowers."

"You mean that Mr. Hoff goes to a different bookstore each day to leave a code message?"

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"That's it. The spy who gets the messages each morning calls him up by 'phone, mentioning just the one word. From that Mr. Hoff knows just where to go, concealing the message in a book before agreed upon."

"The fifth book," interrupted Dean.

"Not always," explained Fleck. "It depends on whether there are five letters in the name telephoned. I have located and copied several more of the messages."

"But who gets the messages he leaves? Who takes them away from the bookshops?" asked Jane, mindful of her own failure in that respect.

"It's a girl, or rather two girls together, though possibly only one of them is in the plot. Very likely the other may not know what her companion is doing."

"To whom does this girl take them?"

"That is still a mystery," said the chief. "We have ascertained who the girl is, where she lives. Her actions have been watched and recorded for every hour in the twenty-four for the last three days, and yet we don't know what she does with these messages. Carter has a theory--tell us about it, Carter."