You may want to check out "Vulcan 607" by Roland White (about the long range bombing mission in the Falklands War). It briefly describes past glories of the Avro Vulcan bomber http://en.wikipedia.org/wiki/Avro_Vulcan including how it was used to test US radar and fighter defences controlled by NORAD.

http://www.amazon.co.uk/Vulcan-607-Rowland-White/dp/0552152293

In the 1960s, the UK tested US radar defences using the Avro Vulcan which was quite a fast, stealthy bomber.

For years it was able to penetrate US defences and successfully "bomb" NY or whatever target in the US was selected for the war games. However, by the late 1970s, it couldn't...

Further confirmation can be found in other histories of the aircraft such as:

http://www.wingweb.co.uk/aircraft/The_Avro_Vulcan.html

By 1958, the Avro Vulcan B.1 was in full service with the RAF as a major component of Britain's nuclear deterrence. Cooperation with the US Air Force's Strategic Air Command (SAC) was close, with the V-bombers and SAC bombers assigned specific roles in joint strike plans. Vulcans often participated in cooperative exercises with their SAC "cousins". Indeed, the RAF maintained a Vulcan service facility at Offutt Air Force Base in Nebraska, the main home of SAC, with Vulcans often dropping in for exercises that ranged over the US and Canada. The Vulcans also often went home with prizes in bombing competitions.

If US air defences could defend against fast, military bombers then it could easily find four airlines flown by amateurs in 2001 especially if they were kind enough to switch off their transponders so they illuminated nicely as "unknowns" on the military radar displays.

To expand upon this a bit: military air defences are tuned to pick up enemy aircraft without transponders. Civil aviation radars are virtually blind without transponders. So the supposed hijackers by switching off the transponders and it did TWO things: make the aircraft invisible to FAA civil aircraft radars - but they wouldn't disappear from the military aircraft radar. The aircraft would shine like a beacon on the military radar! Especially on AWACS radars which would be an entirely separate radar display from NORAD's land based radar system.