Andreas,
The delay in getting the people off of the airplane in Dulles was due to the proper FBI people not being in position. This was not something that could be done by any tom dick and harry, and better to be safe than sorry...And before you ask where was the FBI, remember they are tasked with protecting ALL of America and they are half the size of the NYPD...
If you want to carry out intellegence and use it as a method of defeating terrorists, One of the things you are going to do is develop a list of names. You might not have anything other than the name (they don't submit for a permit to be a terrorist and provide photos and fingerprints in order to become a terrorist).
Unless you want to strip search EVERYONE, (unworkable) there are going to be problems like this. Unless you want blatant Racial profiling this will not be easy, and this sort of thing will happen...
If you want to complain about this, than you have absolutely no right to complain about 9/11 not being prevented.
Cheers
Wino