"When we had a loss of communication and transponder we considered at that point in time a possible hijacking," said Glen Michael, Boston Center air traffic manager.
Within minutes, the Boeing 767 slammed into the north tower of the World Trade Center. But the horror was just beginning.
Aviation officials briefed reporters Monday on their experiences on Sept. 11 at the New York Terminal Approach Control Center on Long Island.
At the New York Air Route center, controllers lost the signal of a second plane. Then they watched as the backup radar showed the westbound United Flight 175 veer sharply off course.
"We tracked that aircraft as it turned southbound then northeast-bound back toward Manhattan. I assumed at that point that that target was in fact the World Trade Center," New York air traffic manager Mike McCormick said.
Controllers tracked the hijacked jet for 11agonizing and helpless minutes.
"For 11 minutes, I knew, we knew, what was going to happen," said McCormick. "And it was difficult."
At FAA headquarters, attention shifted to a third plane, American Airlines Flight 77, flying below radar towards Washington, D.C. As the White House was evacuated, air traffic manager Dave Canoles ordered an assistant to a window.
"I said lets go out to the window and see what you can see," said Canoles. "He came back in pretty much simultaneously with the report of the loss of radar and report that there was smoke coming from the Pentagon."
At the FAA command center, the U.S. aviation system was ordered shut down. At the time, no one knew how many more planes had been hijacked. But controllers had suspicions about 11planes still in the air.
"Certainly after the events took place at the World Trade Center, at the Pentagon, suddenly everything became suspect, so even the smallest anomalies were scrutinized," Canoles said.
United Flight 93 crashed in Pennsylvania 20 minutes after the FAA grounding order. But then it was over. Over the next two hours, 4,500 planes were safely pulled from America's skies – an unprecedented action on an unparalleled day.