http://911woodybox.blogspot.com/2010/11/was-flight-93-part-of-military-hijack.html

Excerpts:

Since 2002, when Jared Israel was the first one to point out the lame reaction of the US air defense to the hijackings (not being able to intercept leastwise one of the airliners), there's still no satisfying answer to the question for those who were responsible for the fatal delays.

...

The Headquarters and the Command Center were tracking Flight 93 as it turned around and headed toward Washington. They got the information that it was hijacked and had a bomb on board. They knew that two airliners had already been used as guided missiles - yet nobody of the leading FAA personnel bothered to follow Cleveland Center's suggestion and tell NEADS (via Dukelin) to scramble fighters. What was going on there?

Additionally, at 9:45 the FAA issued the general grounding order for all aircraft. Who ordered it? The official story has it that it was Ben Sliney, director of the Command Center, but there are also reports that it was Monte Belger, deputy director of the Headquarters - not to forget that Norman Mineta claims the glory for himself. No matter who was it - why did this hero make the risky, unprecedented, precautionary decision to "get all the birds down", but failed to make the acute, urgent, if well-rehearsed and operationally easy decision to alert NEADS about a hijacked airliner on his way to Washington with a bomb on board?

There seems to be only one realistic recourse: Flight 93 was part of a military exercise, probably a hijack simulation, and the exercise plot included a delayed alerting of NEADS. The behavior of Belger, Sliney & co. indicates that they didn't view the plane as a threat, but were told (possibly from the Secret Service who - according to Mike Ruppert - had direct access to FAA data) that it was a "special", i.e. a military exercise flight. The purpose of the delayed alert might have been to test NEADS - to make it more difficult for them to intercept the flight in time.