This primary source comes from the Collection DDE-1001: Christian A. Herter Papers.
Full Citation: State Department Press Release No. 254 Concerning U-2 Incident; 5/9/1960; U-2 (1); Official Correspondence and Memoranda, 1957 - 1961; Collection DDE-1001: Christian A. Herter Papers; Dwight D. Eisenhower Library, Abilene, KS. [Online Version, https://www.docsteach.org/documents/document/sec-state-u2-plane, May 9, 2024]