Zero Fail: The Rise and Fall of the Secret Service by Carol Leonnig:
Steenbergen told Garabito he had been trying to reach him at his office, to warn him of something that the Secret Service couldn’t yet see on Tigerwall.
“We have four aircraft compromised,” Steenbergen began. “Two have hit the towers. Two are headed in the direction of Washington.”
The FAA suspected that these two incommunicado planes veering off course had been hijacked too.
“One is over Cleveland, forty-five minutes out,” Steenbergen said. “Another is over Pittsburgh, thirty minutes out.”