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March 31, 1943

Construction is completed of much of the main technical area, including chemistry and physics laboratories, a cyclotron lab and shop buildings. Today, the "TA-0" site is the area around Ashley Pond in downtown Los Alamos.

Vannevar Bush, director of the U.S. Office of Scientific Research and Development, sends a then-secret memo to President Roosevelt's adviser, Harry Hopkins, on classification of work on "S-1," defending need-to-know principles for collaborations with the British. "The adopted policy is that information on this subject will be furnished to individuals, either in this country or Great Britain, who need it and can use it now in the furtherance of the war effort, but that, in the interests of security, information interchanged will be restricted to this definite objective," Bush writes.

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