Notably, a detection filament placed inside the Mountbatten letter was not restored to its position when the letter was reinserted. When the returned documents were later examined by British intelligence, the absence of this marker confirmed that the letters had been accessed. The Germans had read them.
Kühlenthal considered the material significant enough to travel personally to Berlin to deliver the photographic copies to German military intelligence.
I. WAITING FOR EVIDENCE THAT THE BAIT HAD BEEN TAKEN
In London, the team that had constructed Major Martin waited.
They had done everything within their power to create a convincing identity, to plant the documents in a location likely to produce the right result, and to ensure that the Spanish chain of custody would route the material to German intelligence. What they could not do was control how German analysts would interpret what they received or what decisions German commanders would make in response.
The waiting was partly managed through the signals intelligence that the British collected through the Ultra program at Bletchley Park. The decryption of German military and diplomatic communications had become the most sensitive intelligence asset in British possession, and its existence was among the most closely guarded secrets of the war. Montagu knew about Ultra. He could not refer to it in ordinary documents, but he could receive information derived from it through the proper channels.
On May 14, 1943, roughly two weeks after Martin’s body had been recovered from the Atlantic, a decrypted German signal confirmed that the documents had reached their intended audience. The signal carried the content of German military communications about the expected Allied invasion routes. It stated that Greece and Sardinia were the anticipated targets, with the Dodecanese mentioned as a secondary concern.