UAPs, Clear skies and Radar Intercepts during September 11th attacks

This has occurred to me while listening to the Merged podcast while driving. After the 9/11 attacks, all aircraft were grounded. There was a time in US (and maybe other countries) when there were no authorized civilian or commercial flights. Is it possible to review the radar tapes from that time and identify anomalous objects? Or an opportunity to seek Freedom of Information on military Intercepts of UAPs during that time. It should be a time when the skies are relatively sterile.