http://en.wikipedia.org/wiki/Igor_Gouzenko
Igor Sergeyevich Gouzenko (b.Jan.13, 1919, Rogachevo, USSR - d.June 28, 1982, Mississauga, Canada) was a cipher clerk for the Soviet Embassy to Canada in Ottawa, Ontario. He defected on September 5, 1945, with 109 documents on Soviet espionage activities in the West. This forced Prime Minister Mackenzie King to call a Royal Commission to investigate espionage in Canada.
Gouzenko's defection exposed Joseph Stalin's efforts to steal nuclear secrets, and the then-unknown technique of planting sleeper agents. The "Gouzenko Affair" is often credited as a triggering event of the Cold War...
....When word got out in the media (February, 1946) that Soviets operated a spy network in Canada in which Canadians gave classified information to the Soviet government it created a great stir. Much of the information taken was of public knowledge and the Canadian government was less concerned with the information stolen but more of the potential of real secrets coming into the hands of future enemies.
Canada played an important part in the early research with nuclear bomb technology and that kind of vital information could be dangerous in the hands of other nations.