John Lansdale (1912 - 2003) was a colonel in the US Army.
Lansdale first learned of the Manhattan project from National Defense Research Committee Chairman James B. Conant. At that time, he was assigned to head counterintelligence at the Radiation Laboratory at University of California, Berkeley.
Following the creation of the Manhattan District, Lansdale headed a counterintelligence group in Washington, D.C., which reported to General Groves. Following the decentralization of Army counterintelligence, Groves appointed Lansdale as his special assistant for security. He was consequently in charge of coordinating Manhattan Project security across multiple federal bureaus.
The colonel later served in Europe with the Alsos Mission, at which point he assisted in capturing German scientists and shipping uranium to the United States.