Zero Fail: The Rise and Fall of the Secret Service by Carol Leonnig:
When Scott and Cheney reached the bottom of the stairs in a tunnel leading to the bunker, called the Presidential Emergency Operations Center, Scott still had one major problem. He couldn’t enter the shelter on his own authority. The military tightly guarded access to the PEOC, and unlike top presidential detail agents, many vice presidential agents hadn’t been given the S-keys to get inside.
The Secret Service would later tell the world that Cheney and his detail had reached safety underground, a “secure location,” just a minute or so before the Pentagon crash. They reported that he got to safety by 9:37 A.M.
But the truth of what happened was kept a closely held secret for years. The suspicious hijacked plane crashed into the west side of the Pentagon at 9:38 A.M. At that moment, Cheney was standing at the base of the stairs outside the bunker, powerless and far more exposed if there had been a crash. Cheney had to wait a few more moments for someone to open the door and let him inside. If American 77 had kept heading toward the White House that morning, Vice President Cheney and his detail would more likely have been added to the long list of victims of the 9/11 attacks.