There should have been dozens of cameras around the Pentagon. We were told from investigators who watched the videos that not a single one of them caught the plane crashing into the Pentagon. All we have is a camera that seems to capture only a couple of frames per second. My only question is why not release the security camera footage? It isn't like there is anything classified in the open area surrounding the Pentagon.
The military likes to classify and restrict everything and then keep it secret until otherwise ordered. There’s still numerous classified information, documents, and photos dating back to the Civil War. Tons of secrets date to both world wars.
The paradox is that unless you know the specific secret you can’t declassify it.