FBI's hands were tied.........
By the CIA.
Sure, there was ample evidence that there were some dangerous characters flying around in the US, actually they were flying IN and OUT of the US with diplomatic immunity. And some of the hijackers (Mohammed Atta included) didn't need to learn to fly. They had international pilots' LICENSES, but were busy picking up and delivering international deliveries of heroin to CIA protected fronts. You all need to catch up on your research.
Plausible deniability.
Fat bank accounts.
Age old drug smuggling routes and mules.
Mules are valuable things.
It's easy to look the other way.
The spider finally bit it's keeper........
Shame, shame.
Deny, deny.
Cover up, cover up.