Although airlines have, in the past, been held liable for "ground damages" done by hijacked flights, I think we have to exempt the airlines in this case.
I don't think the airlines (or anyone else) really could have prevented this because nobody expected 757/767's to be used as guided missles. Holding AA/UA liable would simply destroy the 2 largest airlines in the world.
We (the American people) should collectively take up the responsibility of rebuilding the WTC and helping all the victims.
At the same time, a complete moratorium for lawsuits related to this event should be declared. Any lawyer who tries to sue should be labeled as a black-hearted ambulance-chasing money-grubbing b*stard.