"I remember seeing the first tower burning and realizing pretty quickly as the blue sky surrounded that image that every American remembers to this day," recalls Dan Caine.
Caine was like many Americans on 9/11, as he watched the unfolding horror on a television screen.
"I truly realized that we were a nation under attack when I stood there ... and the second airliner hit the second tower. And then, after a second of pause, our training kicked in."
Unlike many of his fellow citizens, he was called to do something about it.
Caine is a colonel in the Air National Guard, 113th Wing, based at Andrews Air Force Base, just outside of Washington, D.C. He flies F-16 fighter jets, and he was ordered on Sept. 11, 2001, to take to the skies to shoot down any other hijacked airlines that posed a threat to the nation's capital. When he got the orders to run out to his jet, which was on standby, he recalls his boss saying, "I don't know exactly what you're going to see out there, I don't know what you are going to face, but I trust you and I got your back."
Those were encouraging words as Col. Caine raced to his jet, climbed into the cockpit as the missiles were loaded onto the wings…for the first time facing a real-life threat on the home front…and not just another practice drill.
"Our mission on the morning of 9/11 was to protect right over downtown Washington, D.C. As my airplane came to life, the generators came on-line, the radios were going ballistic, and you know, people on the emergency channels were saying 'anybody around Washington, D.C., you will be shot down,' and I remember thinking to myself, 'wait...that's me.' "
Caine took to the sky and flew near the Capitol and past the White House.
He never had to fire a missile.
Caine intercepted about a dozen different aircraft, pulling up alongside them on the pilot's left side, waving his wings and indicating the recognized message to change course. He even had to pull in front of some airliners and drop flares out of his fighter, as a signal for the offending planes to turn around.
"Several of the intercepts that I ran were airliners that we pushed away, to tell them to go: 'Man I don't care where you go; you're just going somewhere else now.' So we basically created a zone around Washington, D.C., in which nobody would travel except us…. Our defense in-depth plan became pushing traffic away from Washington out to 40, 50, 60 miles, which created time. The single biggest thing I wanted as the mission commander over downtown, was time. I wanted time to be able to determine what is this airplane's intent, is it under control of the right people that are supposed to be in the cockpit, and so that was our game plan that we executed."
United Flight 93, the Boeing 757 that crashed in Pennsylvania when the passengers fought back against the hijackers, was one of the planes he was told could be a threat.
Col. Caine says "not a day goes by that I personally don't remember the incredible courage, leadership and sacrifice that those passengers made. They made our lives that much more peaceful today by taking matters into their own hands, and being proactive Americans instead of reactive Americans."
Caine spent the day over Washington trying to sort out the mass of airplanes that could have posed a danger, and calls the prospect of having to actually shoot down an airliner, "a complicated question," but realizes that "the nation relies on folks like me to do that and we were prepared to do that, that morning."
F-15 Air National Guard pilot, Col. Timothy Duffy, feels the same way.
"If it came down to that, would we have done it? Yes. If it was legal order and properly authenticated, then we would have done what we had to do. It would have been, obviously, a tremendously difficult thing to do and to live with."
Duffy was scrambled from Otis Air National Guard base on Cape Cod and headed directly to New York City, flying supersonic to get there as fast as he could.
"At 18,000 feet, I really wasn't worried about blowing out anybody's windows. I just wanted to get there as quickly as possible."
At first, he was told the target was a possible hijacked airliner, but as he neared the city, the first plane, American Airlines Flight 11, had already crashed into the north tower.
"That was kind of a shock," he recalls. "We heard about a second aircraft and we didn't know the first one had hit yet. So we were probably something like 80 miles out, going 1.3 to 1.4 mach, and you could see the smoke coming off the towers. As soon as the second aircraft hit, it was pretty obvious...at that point that we're under attack."
Col. Duffy says his first order of business was to make sure there were no more hijacked airliners, and he started a mission similar to Col. Caine's, which was to intercept any questionable aircraft.
He points out that even if he had arrived before the second plane hit, he would not have been able to fire any missiles at the airliner. Those were not his orders.
"If we'd been there earlier, that's probably the question I get the most, if you'd had been there at least in time for the second aircraft, what would you have done? And we would have intercepted it just like we did the other planes. So, you would have seen me pull up beside it and try to turn it away, but that's all that I could do and that's all that we would have had clearance to do."
Duffy intercepted a variety of planes, and when the first tower collapsed, he was doing just that. "When the first tower went down I was actually escorting a Delta jet into Kennedy," Duffy recalls, "and I just remember seeing some, some motion out of the corner of my eye and all I could see was a tan cloud that was over southern Manhattan."
He then headed right for the World Trade Center.
"I flew right up over the top of it and I was looking down, and looking at the square of the roof, really just trying to see if it was leaning or doing anything and it looked good, and I was just getting ready to call and tell them hey, you know the building looks great to me from where I'm sitting, and as I'm looking at the square in the roof it just starts getting smaller. And, you know, for a couple of seconds I really didn't have any frame of reference to put it against, and I wasn't sure what I was looking at until I saw the plume coming out of the bottom, realized it was falling away from me. I have to say that was probably the most horrifying thing for me that day was to witness that."
Duffy says, "I allowed myself that emotional time to kind of be horrified, and then I had to kind of push that out and then get back to business and focus on the task at hand."
The task at hand has remained the same for the National Guard pilots since that morning, a decade ago: Defend America.
"We took off in peace time, and landed, basically, in wartime," notes Col. Duffy.
"We do not get complacent out here," Caine says. "Every time the horn goes off I think this is potentially a similar situation to 9/11."
National Guard units are on standby 24 hours a day, 365 days a year. The fighters are ready to bolt into the sky at a second's notice. And since 9/11, Col. Caine's National Guard unit has scrambled to face reports of a possible new threat, more than 3,000 times.
Just a short walk from the row of fighters waiting to take to the air, there is stark reminder of why Col. Caine and his colleagues are always on alert.
"There's a stone from the Pentagon, reminding us that, again in our history, the sleeping giant has awakened and we should never forget."
Follow Eric Shawn on Twitter: @EricShawnTV