Not all flights were grounded on that day.
https://www.denverpost.com/2006/09/1...m-anniversary/
"While all flights were halted following the terrorist attacks, there was one exception made: The White House authorized planes to pick up 140 Saudi nationals, including 24 members of the bin Laden family, living in various cities in the U.S. to bring them back to Saudi Arabia, where they would be safe. They were never interrogated."