One of the declassified documents outlines the chemicals and techniques necessary for developing certain types of secret writing ink and a method for opening sealed letters without detection.
Notes on verification
Multiple independent outlets (Reuters, The Guardian, NBC) corroborate the CIA's 2011 declassification announcement describing secret ink formulas and covert letter-opening techniques from WWI-era documents.
Sources
- Secret Writing: CIA's Oldest Classified Documents - CIA (cia.gov)
- https://www.reuters.com/article/us-usa-cia-documents/cia-declassifies-oldest-documents-containing-secret-ink-formulas-idUSTRE73D8H020110415 (reuters.com)
- https://www.theguardian.com/world/2011/apr/17/cia-declassifies-oldest-documents-secret-ink (theguardian.com)
- https://www.nbcnews.com/id/wbna42630961 (nbcnews.com)