One day after the attacks, Air Controllers of Washington ARTC Center related to Matthew L. Wald from the New York Times how they spotted an unidentified aircraft that "crossed their radar screen", approaching Washington from the West. They believed it was UA 93.

In contrast, controllers at the Washington Air Route Traffic Control Center had much more warning that something was wrong. Those controllers, who handled American Airlines Flight 77, which dived into the Pentagon, knew about the hijacking of the first plane to crash, even before it hit the World Trade Center, those involved said. That was more than an hour before they watched another hijacked plane, United Flight 93, cross their radar screen on its way to the Pentagon.


http://www.nytimes.com/2001/09/13/national/13AVIA.html


A quick look at an ARTCC map reveals that UA 93 never was in or near the airspace of Washington Center - according to the official story. Even when it crashed in Southern Pennsylvania, it was 50 miles out of this area.

So which airplane crossed the radar screen of the Washington controllers?