Analysis of information sources in references of the Wikipedia article "Konstantin Umansky" in English language version.
The source was apparently quite roundabout. It apparently came to Thomsen from Constantin Oumansky, Soviet ambassador to the United States. Oumansky had received coded information from Under-Secretary of State Sumner Welles that Hitler would soon attack the Soviet Union. Oumansky obviously did not believe the report but did believe that the Japanese security system had been compromised.