In advance of the 10th anniversary of the 9/11 attacks, ABC News interviewed several of the pilots who launched the first airborne patrols over Washington, D.C., and were prepared to target any aircraft that might have posed a threat to the nation's capital. Also interviewed was one of the pilots who escorted Air Force One as President George W. Bush left a school event in Florida bound for a secure location.
On the morning of Sept. 11, 2001, pilots from the D.C. Air National Guard's 121st Fighter Squadron were gathered around a table for a planning meeting when a young intelligence officer entered the room to say a plane had struck the World Trade Center in New York.
Probably an inexperienced pilot caught in bad weather, thought Col. Mark Valentine, who was on his first day on the job as an Air National Guardsmen at Andrews Air Force Base.
Col. Dan Caine thought, "Wow, have I gotten the weather forecast wrong for New York. " As the supervisor of flying that day, Caine was responsible for preparing weather reports.
The pilots continued with their meeting, then took a break to a lounge next door where they watched the events unfold on television that day and began to doubt their initial reactions when they saw the crystal blue skies over New York on television.
"And then, like many Americans, we all remember the graphic image as that second airplane entered that picture and hit the second building," Caine said.
"Right then and there I knew, like all of us did in our country, that we were a nation under attack and at that point we shifted gears and got to work."
Still unaware of the full magnitude of the attacks that day, members of the squadron began to prepare to send aircraft airborne.
Col. Marc Sasseville was one of the first two pilots to go airborne. He recalls not knowing what to expect." We didn't know what was going on, we didn't know who else was up in the air."
Caine took off later to become the mission commander. "We had a 360-degree threat axis because we simply didn't know where any additional airliner might come from. So we spent much of the day intercepting airplanes trying to determine the intent as America's air traffic control system started to shut down and stop. "
He remembers his wing commander telling him before he went airborne, " Dan, I don't' know what you're going to face out there, but you're going to make the right decision and I know you're going to do what's right and I trust you. "
Caine says he intercepted 20 flights that day and says if that if had to encounter any aircraft "all bets were off ... we were clearly a nation at war."
But the first F-16's to go airborne that morning were at a disadvantage because they were not carrying any live weapons because the squadron had just returned from a two-week training mission in Las Vegas.
That posed a problem for pilots who thought they might face the very real possibility of having to shoot down another hijacked airliner targeting Washington. Valentine recalls serious discussions about "what would the best place to ram an aircraft if we had to knock it down. "
"Looking back, you wonder, 'Wow, how could someone honestly be thinking about that?' But at the time we actually had to discuss that. Thankfully, we never had to do that."
Other F-16's that went airborne later that day from Andrews were able to be armed with live weapons.
According to Sasseville, "we slowly figured things out, figured out that NORAD had sent airplanes into the air as well. We started talking to each other. It was very confusing not knowing who was who, trying to intercept airplanes and try to get them turned away from the national capital region."
He described the day as a being a "kind of a roller coaster" between carrying out the intercepts and looking back to see smoke still smoldering from the Pentagon.
"It was a horrible, horrible feeling to experience looking at the Pentagon on fire," he said. "That's one of the memories that I'll keep with me."
Valentine says that shortly after the first planes went aloft, squadron members quickly realized hey had to plan to have planes over Washington for the long term.
Valentine flew his first combat mission the next day and he, too, recalls seeing the smoke still billowing from the Pentagon, but he did not realize until much later how eerily quiet the skies over Washington had become.
"I've flown around the D.C. area the last 10 years and it's a busy place and I didn't realize how eerie flying during the Sept. 11 period was until after it was over and normal operations resumed," he said. "Because there was nothing in the air besides us."
Valentine said 9/11 definitely changed his outlook about what's possible in the world after having witnessed the events of that day first hand. "To see the Pentagon burning underneath you and hearing planes in the sky with radio calls saying you will be shot down. Those are not normal things you hear in America. So I think it expands your realm of the possible."
Sasseville said, "at the end of the day we were under attack and this is going to change our country for a long time and we really need to pull together as a country "
Within weeks ,Caine soon found himself deployed to Kuwait where he flew combat missions over Afghanistan.
The lessons from that day have led to the alert scramble process now in place where dedicated armed fighter aircraft are always minutes away from launching intercept within minutes. Since 9/11, the D.C. Air National Guard has launched 3,000 alert scrambles.
In Houston that morning, Lt. Col. Rolando Aguilar was asleep in the cockpit of his F-16 as the Texas Air National Guardsmen completed a 72-hour alert shift intended to cut response times for getting fighter jets into the air.
He was awoken by the sound of scramble horns alerting him to fly an alert mission. It wasn't until then that he found out that two aircraft had struck the World Trade Center.
Aguilar and his wingman received vague orders to fly to a location over the Gulf of Mexico. Just before he took off, he received a cryptic message from his wing commander that he was going to intercept a former member of his unit, but Aguilar still had no idea what mission he was to conduct.
When he reached the intercept location, he was startled to see a Boeing 747 aircraft with the markings identifying it as Air Force One. "Wow," he said. He recalls thinking, "Hey, this is no kidding the real deal. "
A confused Aguilar radioed his controllers that he had met up with Air Force One. "They don't tell you anything until we said it's Air Force One, and they say, 'Well, yes. That's your target. Escort it.'"
It was then that Aguilar recalled that President George W. Bush had been a member of his Texas Air National Guard unit.
Aguilar said he and his wingman did not know where Air Force One was flying, but they positioned their aircraft in an escort formation with Aguilar several miles ahead of the president's plane checking the radar for any possible threats.
He speculated that given their altitude and speed, the only thing that could catch them would be another fighter jet, but he made sure he didn't miss anything on his radar screen.
"It was just a little humbling to be trusted with that responsibility, flying next to Air Force One with live missiles and hot guns," Aguilar said.
Air Force One eventually landed at Barksdale Air Force Base in Louisiana and Aguilar provided air cover over Barksdale until Air Force One took off for Offutt AFB in Nebraska before returning to Washington.
He says now that he was prepared to shoot down any aircraft that might have targeted Air Force One while it was in the air, but he thinks now he would have likely carried out a proportional response, striking an aircraft engine so the passengers aboard could have landed safe. "You feel conflicted, you have to do what needs to be done for your country," he said.
Of the events that day, Aguilar said, "it was an event that changed the world, it impacted my life tremendously."
Aguilar has served four deployments over Iraq. "It's just doing my duty, serving as a soldier, as an airman," he said, "going where I'm called to."