Then he collected the coffee cups and withdrew.

My attention, diverted for a moment, came back to Ackroyd. He was staring like a man turned to stone at a long blue envelope. The other letters he had let drop to the ground.

“ Her writing ,” he said in a whisper. “She must have gone out and posted it last night, just before⁠—before⁠—”

He ripped open the envelope and drew out a thick enclosure. Then he looked up sharply.

“You’re sure you shut the window?” he said.

“Quite sure,” I said, surprised. “Why?”

“All this evening I’ve had a queer feeling of being watched, spied upon. What’s that⁠—?”