In 2011, the CIA declassified the US Government's six oldest classified documents, dating from 1917 and 1918.
Notes on verification
Confirmed by CIA's own account and multiple independent contemporaneous news reports (NPR, Washington Post) from April 2011.
Sources
- Secret Writing: CIA's Oldest Classified Documents - CIA (cia.gov)
- https://www.npr.org/sections/thetwo-way/2011/04/07/135227704/cia-declassifies-its-oldest-documents-secret-ink-formulas (npr.org)
- https://www.washingtonpost.com/blogs/checkpoint-washington/post/cia-declassifies-its-oldest-documents/2011/04/07/AFCLwYtC_blog.html (washingtonpost.com)