I've been thinking about this a lot and it could probably have been a lot worse.
Now gettng a plane full of fuel and crashing it is only gonna cause say 50.000 - 100.00 deaths at a guess and there ain't that many targets u can get that many ppl, especially after usa is on full alert.
For the above they would prolly need ppl on the inside like stewardesses or pilots, stewardesses etc are more likely since prolly less security checks and easier to get the job etc. I'm guessing stewardesses don't get many security checks either.
Now what they could have done is smuggeled small nukes (u can fit a good nuke in say 2 back-packs, detonate it in say subway and big trouble. You would want to smuggle this in via say boat or something, usa will prolly up seceurity in harbours etc now.
Lastly is biolgical, whcih is prolly a lot easier to smuggle in and will cause a lot more damage, they could prolly make it in the usa, from what I know it isn't that hard to make say anthrax.
Let's just hope this don't get worse cos it don't take much for things to escilate, they nuke the usa, usa nukes back, nuculars subs start nuking etc and no more life on earth within a few years.