“Don’t step there, Walter,” he cautioned.  “Step over the rug.  I’m expecting visitors.  How was she when she arrived home?”

I told him of her parting injunction.

“Not bad advice,” he remarked.  “I think there’s a surprise back of those warnings.  They weren’t sent just for effect.”

He had closed the door, and we were standing by the table, looking at the letters, when we heard a noise at the door.

It was Norton again.

“I’ve been thinking of what you told me last night,” he explained, before Kennedy had a chance to tell him to step over the rug.  “Has anything else happened?”

Kennedy tossed over the anonymous letter, and Norton read it eagerly.

“Whom does it mean?” he asked, quickly glancing up, then adding, “It might mean any of us who are trying to help her.”

“Exactly,” returned Kennedy.  “Or it might be Lockwood, or even de Moche.  By the way, you know the young man pretty well, don’t you?  I wonder if you could find him anywhere about the University this morning and persuade him to visit me?”

“I will try,” agreed Norton.  “But these people are so very suspicious just now that I can’t promise.”

Norton went out a few minutes later to see what he could do to locate Alfonso, and Kennedy replaced another blank sheet of paper for that under the rug on which Norton had stepped before we could warn him.

No sooner had he gone than Kennedy reached for the telephone and called Whitney’s office.  Lockwood was there, as he had hoped, and, after a short talk, promised to drop in on us later in the morning.

It was fully half an hour before Norton returned, having finally found Alfonso.  De Moche entered the laboratory with a suspicious glance about, as though he thought something might have been planted there for him.

“I had a most interesting talk with your mother yesterday,” began Kennedy, endeavouring by frankness to put the young man at ease.  “And this morning, already, Senorita Mendoza has called on me.”

De Moche was all attention at the words.  But before he could say anything Kennedy handed him the anonymous letter.  He read it, and his face clouded as he handed it back.

“You have no idea who could have sent such a note?” queried Craig, “or to whom it might refer?”

He glanced at Norton, then at us.  It was clear that some sort of suspicion had flashed over him.  “No,” he said quickly, “I know no one who could have sent it.”

“But whom does it mean?” asked Kennedy, holding him to the part that he avoided.

The young man shrugged his shoulders.  “She has many friends,” he answered simply.

“Yes,” persisted Kennedy, “but few against whom she might be warned in this way.  You do not think it is Professor Norton, for instance—­or myself?”

“Oh, no, no—­hardly,” he replied, then stopped, realizing that he had eliminated all but Lockwood, Whitney, and himself.