Seventy years ago, on 9 September 1949, Director of Central Intelligence Admiral Roscoe Hillenkoetter handed President Harry Truman a carefully worded report of “an abnormal radio-active contamination" in the Northern Pacific that greatly exceeded normal levels in the atmosphere.  While uncertain as to the cause, the DCI’s first hypothesis was “An atomic explosion on the continent of Asia.”  This proved to be accurate – it was the first Soviet test of a nuclear device.

Moscow’s success in building a nuclear bomb was a monumental development made all the more alarming for U.S. strategists by the fact that it occurred one-to-four years sooner than analysts had expected.  The White House chose to preempt possible Kremlin triumphalism by announcing the finding to the world on 23 September 1949, a move that evidently came as a shock to the Soviets who had no idea the U.S. had the capability to isolate and identify the signs of a nuclear blast.

Hillenkoetter’s memo, never before published, is at the core of a new posting today by the National Security Archive offering previously classified information and context surrounding the U.S. discovery of the landmark Soviet test.  The documents are an update to an earlier Archive compilation and focus on the state of U.S. intelligence about the Soviet nuclear program before and after the test.  They help address lingering questions about the unexpected abilities of U.S. nuclear detection technology but also about the disturbing failure to predict the Soviet atomic breakthrough more accurately.