The database of documents includes more than 1,500 pages of UFO-related material from the Advanced Aerospace Threat Identification Program (AATIP) — a secretive U.S Department of Defense program that ran from 2007 to 2012.
Link to Sun article from LiveScience.
Despite never being classified as secret or top secret, the AATIP only became known to the public in 2017, when former program director Luis Elizondo resigned from the Pentagon and released several now-infamous videos of an unidentified aircraft moving in seemingly impossible ways to the media.
Shortly after the AATIP's existence was revealed, the U.S. outpost of the British Tabloid The Sun filed a FOIA request for any and all documents related to the program. Four years later — on April 5, 2022 — the U.S. Defense Intelligence Agency (DIA) honored the request by releasing more 1,574 pages of material to The Sun.