It's very suspicious that F-16s were scrambled from Langley to protect Washington from attack, and not Andrews Air Force Base, which was much closer.
According to the 9/11 Commission, Andrews had no available fighters on alert that morning..
"All the hijacked aircraft were in one of NORAD’s Continental U.S. sectors, the Northeast Air Defense Sector (also known as NEADS). NEADS is based in Rome, New York. On 9/11, it could call on two alert sites, each with one pair of ready fighters. These were the 2 Otis Air National Guard Base in Cape Cod, Massachusetts and Langley Air Force Base in Langley, Virginia".
9/11 Commission Staff Statement 17
Some sites say this is hard to believe, because it’s known that Andrews had “combat ready” fighters.
"On 11 September there were two entire squadrons of combat-ready fighter jets at Andrews. Their job was to protect the skies over Washington D.C. They failed to do their job. Despite over one hour's advance warning of a terrorist attack in progress, not a single Andrews fighter tried to protect the city".
However, the term "combat ready" does not mean "available to launch within minutes" on an emergency intercept. It's a more general term than that. For example, the 180th Fighter Wing page tells us that:
"The mission of the 112th Fighter Squadron is to provide combat ready aircrews capable of deploying anywhere in the world within 24 hours of notification".
Airman (the “magazine of America’s Air Force”) offers some confirmation that Andrews wasn’t always on alert, at least when this article was published back in December 1999:
The Air National Guard exclusively performs the air sovereignty mission in the continental United States, and those units fall under the control of the 1st Air Force based at Tyndall. The Guard maintains seven alert sites with 14 fighters and pilots on call around the clock. Besides Homestead, alert birds also sit armed and ready at Tyndall; Langley AFB, Va.; Otis Air National Guard Base, Mass.; Portland International Airport, Ore.; March ARB, Calif.; and Ellington Field, Texas.