While at American universities in early 1933, he undertook his third two-month visiting professorship at the California Institute of Technology in Pasadena. In February and March 1933, the Gestapo repeatedly raided his family's apartment in Berlin. He and his wife Elsa returned to Europe in March, and during the trip, they learned that the German Reichstag passed the Enabling Act, which was passed on 23 March and transformed Hitler's government into a "de facto" legal dictatorship and that they would not be able to proceed to Berlin. Later on they heard that their cottage was raided by the Nazis and his personal sailboat confiscated. Upon landing in Antwerp, Belgium on 28 March, he immediately went to the German consulate and surrendered his passport, formally renouncing his German citizenship. The Nazis later sold his boat and converted his cottage into a Hitler Youth camp.