The Defense Stand-Down: Dozens of times every year for decades,
U.S. jet fighter planes were rapidly launched ("scrambled") to
check out airliners and other planes that were out of communication,
off course, had their radar transponders turned off, etc. This was
always done within 5 to 10 minutes of learning about the problem.
On 9-11, after four airliners were known to be hijacked, the launch
of jet fighters was delayed for 75 minutes! And when they were sent
up, they flew at only 500 mph, one third of their maximum speed!

A few months before 9-11, Secretary of Defense Donald Rumsfeld
issued an order to the Air Force that no jet fighters were to be
scrambled except by his personal orders.