Surveillance
It was later known that von der Osten did arrive in the U.S. with the purpose of directing the Joe K spy ring; however, with his untimely death, Ludwig was left in command of the ring. Under FBI surveillance Ludwig was seen continuing his routine of visiting the docks in New York Harbor and U.S. Army posts around the state. On one occasion during May he took an extended trip to Florida, accompanied by Lucy Boehmler who acted as his "secretary" and courier who assisted him in preparing his reports and in maintaining detailed records of his observations by using a toothpick dipped in invisible ink. They stopped along the way at Army camps, airfields, and factories engaged in manufacturing wartime matériel.
When he arrived in Miami, Florida, Ludwig met up with an agent, Carl Hermann Schroetter, and through Schroetter Ludwig was able to send his reports on his observations during the trip, including the progress in the construction of a naval base in Miami.
Read more about this topic: Kurt Frederick Ludwig