The attack on 911 opened our eyes that we were vulnerable to an attack from terrorist. This was an awakening that there are those who hate Americans and want to do harm to us.
At that time the actions of the terrorist was to kill as many Americans as possible and cause as much disruption as possible.
In some ways they succeeded but American came back. With their current means they are only able to attack and cause harm in localized areas such as shopping malls, buildings, airplanes, etc. If they are successful in terrorizing Americans with their current capabilities the disaster, though horrific, does not necessary contemplate a high level disaster.
There are other possibilities on how they may attack again. Most likely they will attack similar as before on several venues. However, it is quite possible that they will attack in trying to disrupt our energy sources such as electric grid via a cyber attack, or blow up a nuclear generating facility.
If their means and methods of attack advance to such a level then they could cause a disaster of a higher magnitude possibily on a regional level. This would be an event that your Emergency Preparedness Plan should be able to help you survive.
During an event such as this you will may food shortages and rioting on a local or regional scale.
In several years it may be possible for terriorist to obtain a nuclear device which could be denotated to destroy a widespread area in a heavy populated area or attempt to create an EMP event. Such an action would result in a level 2 or 3 event but the time factor may be longer.