Well 1st, it is part of the FAA mandate to keep records of all incidents and accidents/ hijackings. That is part of their mission.

You can find other hijacking events on their website faa.gov

Next, the FAA was a defendant in the case. FAA personnel testified at the trial, gave depositions, etc. The DOT Secretary and FAA D director of Safety were also named.

This was the 1st hijacking in the US where a captain had been killed.

The records should be maintained in their archives.....everything they do gets stored there from since the agency was created.