11. The handkerchief and the invisible ink
Each of the team leaders – Dasch and Kerling – were supplied with a list of contacts, German agents in the United States who could be turned to for help. The lists were written in an innocent-looking handkerchief in invisible ink. Both leaders were taught how to reveal the lists during training, but Dasch had been an indifferent trainee, and he could not recall how to expose the lists of contacts. The FBI sent the handkerchief to its crime lab for analysis. It did not take long for the technicians to discover that exposure to ammonia unveiled the writing, and the FBI soon had all of the contacts under 24-hour surveillance.
Kerling and Thiel had been in Cincinnati, where they made contact with German agents, before traveling on to New York. Kerling and Thiel went on to New York by train, where Kerling contacted a friend named Helmut Leiner, who was on the list provided by Dasch’s handkerchief and consequently under FBI surveillance. Kerling wanted to borrow one of Leiner’s mistresses during his stay in America. The FBI spotted Leiner and Kerling, and followed the latter to a bar where he met with Thiel. Both were immediately arrested. Six of the conspirators were then in custody. The only remaining saboteurs were in Chicago, though the FBI did not know that as yet.