Interesting extract from the link provided above by CarolC

Letter 1st June, 2001 - section this extract is taken from relates to civilian aircraft hijacking within FAA controlled airspace.

3. Procedures

a. General.
Military personnel may not participate in a search, seizure, arrest, or other similar activity. This restriction would include the apprehension of aircraft hijackers or use of military aircraft (fixed-wing or helicopter) or other vehicles as platforms for gunfire or the use of other weapons against suspected hijackers.