The U.S. Army Black Hawk helicopter that collided with a ...

The U.S. Army Black Hawk helicopter that collided with a passenger jet in Washington, D.C., was part of an elite "Doomsday readiness" unit responsible for evacuating top U.S. officials in a national crisis.

The Black Hawk helicopter was preparing for its role in transporting leaders to secure locations if the Capitol were under attack.

This secretive unit has a long history of critical operations, including emergency flights on 9/11.

More simply, it is known as "continuity of government."