where were they?

Washington DC is heavily guarded by what exactly?

What radars are there?

What are the capabilities of those radars?

Where are they located?

How much warning did the North East Air Defense Command have that Flight 77 had been hijacked?

What is the response time of the air defense system?

What were the Rules of Engagement in place on 9/11/2001?

Tell you what, I will save you the trouble of looking up the answers to these basic questions you should have already asked. American continental air defense in September 2001 was designed to protect us from EXTERNAL threats. This is why the terrorists chose to hijack domestic airliners. They also hijacked the planes close to their targets to minimize the response time of the air defense. Additional the hijackers turned off the aircraft transponders and performed maneuvers and course changes that would make them more difficult to track.

Contrary to popular conspiracy theorist internet fantasy, there are no special radars and no missile batteries protecting Washington DC and there have not been since the early 1970's. The Pentagon is in fact directly under the flight path for Ronald Reagan airport and hundreds of aircraft fly just a few hundred feet over the building every single day. CONUS air defense relies on early warning radars that look beyond our borders linked to command centers which can scramble fighters that are on 15 minute alert (meaning they can be airborne in 15 minutes).

NEADS had 9 minutes warning that Flight 11 was hijacked before it hit the North Tower. Flights 175 (South Tower), 77 (Pentagon) and 93 (Shanksville) all crashed BEFORE anyone at NEADS was aware they were hijacked or possible hijacks. For fighters on 15 minute alert that may be hundreds of miles from the hijacked aircraft, how quickly can they intercept a plane that has already crashed?

Even if they did intercept the plane, so what? Do they automatically just shoot down a plane full of possibly hundreds of American citizens and potentially over a densely populated area because they think it might have been hijacked and it might be used as a weapon? Even today it requires a specific order from the President to shoot down a suspected hijacked plane.

the plane got through before everything was guarded so well and a plane attack was never expected