When American Airlines Flight 11 crashed into the World Trade Center, word spread quickly to the Air National Guardsmen at Joint Base Andrews.
The group of pilots assumed bad weather must be to blame -- but TV coverage showed a clear blue sky behind the Twin Towers.
That's when United Airlines Flight 175 crashed into the South Tower.
"Right then and there we knew, and I knew as a tactician, that we were a nation at war, that we were under attack," said Col. Dan Caine, Air National Guardsman.
Former President George Bush was at an elementary school in Florida that morning. When Air Force One took off, F-16 pilot Rolando Aguilar was assigned to escort and protect the Commander-in-Chief.
"Make sure I'm not missing anything - being extra vigilant - looking at radar scope, making sure no one is popping up that should be," Aguilar said.
Then a third hijacked plane - American Airlines Flight 77 - crashed into the Pentagon. Another F-16 pilot, Billy Hutchison, was training over North Carolina -- and told to immediately return to Andrews.
As a fourth group of hijackers flew United Airlines Flight 93 toward the District, Hutchison was sent to investigate and he prepared for the worst.
"To go after a civilian aircraft with innocent civilians onboard is not what we train to do," he said. "That will sit in back of mind, but devastation that could come aircraft reaching target on ground could be far greater."
But then the crew and passengers of Flight 93 charged the cockpit and the plane crashed in Shanksville, Pa.
Their act of courage hasn't been forgotten.
Since that day, the Air National Guard has changed. the pilots are on 24-7 alert.
In ten years, they've faced 3,300 "alert events" and they can scramble and be in the air in just minutes to defend our nation's capital.