All of the U.S. government’s files on the assassination of President John F. Kennedy are supposed to be released by October 26. But one batch of CIA records on suspected assassin Lee Harvey Oswald has gone missing.

The records were part of a seven-volume file on Oswald held by the agency’s Office of Security. The OS is responsible for protecting CIA property and vetting agency personnel, and maintains a file system independent of the CIA’s Central File Registry. Declassified CIA records show that Volume 5 of the file records existed as recently as 1978.

The disappearance of the records, discovered by JFK researcher Malcolm Blunt, is significant because the Office of Security was the first component of the CIA to open a file on Oswald, an ex-Marine who defected to the Soviet Union in October 1959 and was later charged with killing JFK in Dallas on Nov. 22, 1963.