USSS

[...]Thru monitoring radar and activating an open line with the FAA, the Secret Service was able to receive real time information about other hijacked aircraft. We were tracking two hijacked aircraft as they approached Washington, D.C. and our assumption was that the White House was a target. While the White House was evacuated, the Secret Service prepared to defend the facility.[...]

http://pccw.alumni.cornell.edu/news/newsletters/spring06/riggs.html

[...]on 9/11, the Secret Service had “a system that allowed them to see what FAA’s radar was seeing[...]
Against All Enemies: Inside America's War on Terror
by Richard A. Clarke (pag. 7)