On 21 May 1915, Rotterdam Head Commissioner A.H. Sirks personally questioned the 64-year-old German national Franz Leibacher or Laibacher at Rotterdam Police Headquarters (image). Leibacher received letters from German spies in the United Kingdom, such as Karl Müller, John Hahn and Anton Küpferle, who the previous day had hanged himself in his British prison cell. Müller would be shot in the Tower of London. After Leibacher's cover was blown to MI5 by an unknown informant, they were caught. Sirks wondered why Leibacher had false Belgian identity papers. He was doing nothing illegal in the neutral Netherlands. Leibacher belonged to the circle of German secret agent Martin Wilhelm Rehder.