They may not have a 9/11 style attack in mind. They might simply want to bring down an air liner.
If they wanted to down a plane there are more than enough Islamists working at major airports. Won’t have to board the plane to do that - political correctness and all that.
I am thankful that they have not done a mass mailing of incendiary devices that would be shipped by air. I am still surprised they have not hit a train, bus, or sporting event.