There was a video at the time of a woman in Shanksville. She saw a plane approach low, from the South I think. It was the opposite direction it should have been. What she described sounded a lot like a predator drone though the general public didn’t know what those were for several more years.
Here’s a short vid of a guy that does optical and digital zoom-ins on chemtrailing planes. He figures they are small (maybe 6-8 ft) but look like full size planes. By extension, they aren’t that high either.
https://youtu.be/_kB13tJyYIo
Could be multiple types of craft spraying and doing other “activities.” Opens up a bunch of possibilities to consider.