C-byrån ("C-bureau") was a Swedish secret intelligence agency established in 1939, sorting under the Swedish Armed Forces. It was led by Major Carl Petersén. During World War II C-byrån organized operations in the German-occupied Norway and Operation Stella Polaris in Finland.

Women that had very dangerous missions as spies at C-Bureau were called "swallows" or "svalor" in Swedish. Erika Schwarze was a young German girl recruited here, which later became one of the most important spies of World War 2, she later received a nickname "Onkel".

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https://en.wikipedia.org/wiki/C-byr%C3%A5n