Who issued the order to shoot down Flight 93?

With the Capitol under attack, the President on Air Force One, and the Defense Secretary missing, Vice President Dick Cheney was the most powerful man in the country on the morning of 9/11. From a bunker beneath the White House, he gave fighter pilots flying above the Capitol permission to shoot down hijacked commercial planes.

But did Cheney actually have the authority to issue this order? Did he and President Bush connect before or after Cheney ordered the fighters into battle? And was the U.S. even ready to shoot down any hijacked flights?