Warning: The following content may be distressing for some viewers.Hundreds of New York firefighters and police officers immediately headed to work once Flight 175 hit the South Tower. One of those officers was Louis Savelli.After leading the NYPD counter-terrorism unit created in the aftermath of the attack for many years, the born-and-bred New Yorker eventually moved his family to Iowa for a fresh start, opening an authentic pizzeria in 2014. He brought a little piece of the Big Apple with him: posters, pizza ovens and his prized accent. The retired detective came to Camp Dodge with lessons learned from 9/11 to teach counter-terrorist training to law enforcement all over the country."That was my unit... to arrest members of Al-Qaeda," Savelli said. "When the second plane hit, which I knew right away, and then everybody knew right away that was a terrorist attack. I ran right into work," Savelli said. It was a life-changing day burned into his mind. "The streets of New York were mayhem," Savelli said. "People were trying to drive all over to get places. Nobody was stopping at lights. Horns were beeping. It was something out of a movie, like a science fiction movie."Savelli shared photos that he had captured from ground zero. "It was just constant dust for quite a while, from all the debris, from the pulverization of the buildings, all the stuff that was in the air. It was very surreal," Savelli said. He and his team worked frantically. "Trying to sift through tons and tons of rubble by hand, trying to find people. And then just being frustrated not really finding anything," Savelli said. However, there were only a few survivors. "A lot of memories of specific things... are kind of, like, blurry or nonexistent," Savelli said. "So a lot of things that happened, it's hard to remember specific things. But I do remember my recovery efforts at ground zero were mostly body parts. It's kind of hard to say on TV. I don't remember an entire person, finding an entire person on that day, on 9/11, and then throughout that time. I assume that ... not remembering is a part of my mind blocking it out." Now, 20 years later, he cannot block it out. Like many of the first responders, Savelli is suffering. The dust he breathed in at ground zero has caused asthma, skin cancer and gut-wrenching stomach pain now. "Sadly, all those guys that stayed there looking for people and looking for DNA, and anybody who could've at least recovered something for the family, every one of them has gotten seriously sick," Savelli said. "At least 5,000 total first responders have died of cancer. And about 15,000, 20,000 have gotten some sort of serious cancer-related disease since then."In addition to the pain is the fear that terrorism will rear its ugly head on United States soil again after pulling out of Afghanistan. "Without a doubt, we are much more at risk today than we've been in a long time," Savelli said. "Are we going to have people like Taliban or ISIS or Al-Qaeda or Haqqani Network or some others coming in with them? And without a doubt that will happen. So we have to do a better job." Aside from his pizza businesses in Des Moines, Savelli also runs a security firm."I was a member of the NYPD on 9/11 at ground zero and you know we did our job, but the United States military sacrificed so much to go overseas and protect this country," Savelli said. "And they are my heroes." He said we should remember that the people willing to put their lives on the line, don't do it for the money. "The people out there sacrifice, for very little money. The cops, the firefighters, the military, EMTs, they make very little money," Savelli said. "But yet they risk their lives out there for strangers."

For John Wesley, no day has a greater significance than Sept. 11, 2001. His reasons are deeply personal, powerful and inspiring.Images of his fiancée are indelibly etched in Wesley's mind, heart and soul. Sarah Clark died when terrorists hijacked American Airlines Flight 77 and deliberately crashed into the Pentagon."She saw the same old world each day with new eyes, and her compassion never blinks," Wesley said.Authorities allowed Wesley to watch the boarding gate video. He saw two of the hijackers looking at a child playing with an Elmo toy, seniors being pushed onboard in wheelchairs and students securing their backpacks."For me, how could you look at that and still do what was on your mind?" Wesley said.Wesley decided at the last minute not to go on the trip with Clark because it was his first day on the job as an actor on HBO'S "The Wire.""Growing up in Mississippi, I just believe I would have fought," Wesley said.Every year on the 9/11 anniversary, Wesley visits the Pentagon crash site. He will be there this year, as well. In 2018, former Vice President Mike Pence mentioned Clark in his remarks. Wesley said her positive influence has made him a better man. He said she helped lead him back to his faith."People need help. People need compassion. And I wanted people to know that I had no malice, that I never asked God why. I just thanked him for the time that I had with her and all the things that happened with her," Wesley said.Clark taught sixth grade at Backus Middle School in Washington, D.C. She was on Flight 77 to chaperone students to an ecology conference sponsored by National Geographic.Days before the flight, Wesley and Clark decided to bump up their wedding date to December. They picked a place to hold their wedding reception and shopped for wedding bands.Wesley had the grim task of identifying Clark's remains."I was looking for this ring because this is the ring she would have had on," Wesley said. Wesley said that over the years, he has focused on writing music and books inspired by Clark."If we are going to stop this hatred, we are going to have to start with the children," Wesley said. "That's the real lesson, and that will be our saving grace if we learn to love each other."Wesley has since found a new love, which he said has helped him emotionally. His work in the Baltimore City Office of Civil Rights is his passion."It seems I am where I'm supposed to be," Wesley said.Wesley has advice for anyone who has suddenly lost a loved one, saying life is short, to honor them by doing a simple act of kindness and use that depth of pain to lift others up.

When this century began, you could pull up to the airport 20 minutes before a domestic flight in the United States and stroll straight over to your gate. Perhaps your partner would come through security to wave you goodbye. You might not have a photo ID in your carry-on, but you could have blades and liquids.Back in 2001, Sean O'Keefe, now a professor at Syracuse University and former chair of aerospace and defense company Airbus, was deputy director of the Office of Management and Budget in the George W. Bush administration."At the White House, I was a member of the National Council Security team," he told CNN Travel. He and his colleagues had been briefed on the al Qaeda terrorist group and understood the threat it posed, "but at the same time our imaginations simply did not give us the capacity to think that something like could happen."It had been nearly 30 years since Palestinian terrorist attacks at Rome airport in 1973, which killed 34 people and demonstrated that air travel was vulnerable to international terrorism. "That seemed to have changed the whole security structure in Europe and in the Middle East in a way that didn't really penetrate the American psyche," O'Keefe said. "It's this typical American mindset; we have to experience it to believe it."Then on the morning of Sept. 11, 2001, a team of 19 hijackers was able to board four different domestic flights in the northeastern U.S. in a series of coordinated terror attacks that would claim 3,000 lives. Flying in America, and the rest of the world, would never be the same again.'Something just happened in New York City'O'Keefe was in the White House's West Wing with Vice President Dick Cheney when the news came through. They "had the television on, matter of fact it was CNN," he recalled. "The phone rang. His receptionist was on the hotline to tell him to (turn the sound up); something just happened in New York City."Like millions of people around the world watching the same scenes live after the first plane hit the World Trade Center's North Tower, O'Keefe and his companions assumed they were witnessing a terrible accident, a matter for the Federal Aviation Administration and the Department of Transportation.But when the second plane hit the South Tower 17 minutes later, O'Keefe said, "That was the moment where it was really evidence that this was something more than an accident, this was a premeditated effort. The security guards, the Secret Service, all mobilized."The events of that morning in the U.S. changed the nation "automatically, immediately, into one obsessed, in big ways and small, with protecting its security," wrote historian James Mann in 2018. "The way that 325 million Americans go through airports today started on Sept. 12 and has never gone back to what it was on Sept. 10." 'We all had an epiphany on the same day'The U.S. government immediately began work on the security manifesto that by November 19, 2001, would be passed into law as the Aviation and Transportation Security Act."The fact that they had orchestrated that strike with three different flights in three different places" made clear how vulnerable the U.S. was, O'Keefe said. "That was a real slap in the face. It reminded us how naive we had been."Getting agreement from Congress on security changes was fast and unanimous, he recalled. We needed "to make the resources available right away, to reinforce all those doors and cockpits (and) actually establish security perimeters."In airports and on airlines, meanwhile, tougher security measures were introduced as soon as civilian air travel resumed on Sept. 14. The National Guard provided armed military personnel at airports, and travelers faced long lines as the new systems got their start.Those early post-9/11 passengers -- people who hadn't canceled or rescheduled their trips -- were, O'Keefe said, largely accepting of the new high-security regime, with its disruptions and delays. "We all had an epiphany on the same day."Identification checksSome of the 9/11 hijackers had been able to board flights without proper identification. After the attacks, all passengers age 18 and over would need a valid government-issued identification in order to fly, even on domestic flights. Airports could check the ID of passengers or staff at any time to confirm that it matched the details on their boarding pass.Before the events, the U.S. federal government had a small list of people deemed a threat risk to air travel. However, what we know today as the No Fly List -- a subset of the Terrorist Screening Database denoting people who are barred from boarding commercial aircraft for travel into, out of and inside the U.S. -- was developed in response to 9/11.Around the world, countries became more stringent with identity checks, security screening and their own versions of the No Fly List. In 2002, the European Union introduced a regulation demanding airlines confirm the passenger boarding the aircraft is the same person who checked in their luggage, which meant checking ID both at luggage check-in and when boarding. Later in the decade, fingerprint IDs and retina and iris scanning were introduced in some countries.The creation of the TSAAirport screening in the U.S. used to be piecemeal, undertaken by private security companies appointed by airlines or airports.As part of the new security act, the Transportation Security Administration (TSA) was introduced in November 2001. Now an agency of the U.S. Department of Homeland Security, which was formed a year later, the TSA took over all the security functions of the FAA and U.S. airlines and airports.By the end of 2002, the agency had already recruited close to 60,000 employees, wrote TSA historian Michael P. C. Smith.Looking back 20 years later, O'Keefe reflected that it was "an enormous challenge in that immediate time afterward to mobilize a whole new cadre of security forces, thousands of trained professionals to do this.""It was not without its flaws," he added. "Recruiting issues and right training and all the things that were necessary: We went through plenty of fits and starts to make that happen."The fact that America's "allies and friends and partners" around the world "had already been through this," was a huge benefit, he said. "We were able to learn from them, how they did it and what they did."Security screeningSome of the 9/11 hijackers were reported to have been carrying box cutters and small knives, which they were able to bring through security.Before long, with the new streamlined enforcement by the TSA, potential weapons like blades, scissors and knitting needles were no longer allowed on board, and airport workers were better trained to detect weapons or explosives.By the end of 2002, the TSA met a key mandate of the Aviation and Transportation Security Act by deploying explosives detection systems nationwide. In the following years, other terrorist attacks would further change what we could and could not bring on board planes.In August 2006, a foiled plot to detonate liquid explosives on multiple transatlantic flights led to today's restrictions on liquids, gels and aerosols in carry-on luggage. That same month, the TSA began requiring passengers to remove their shoes to screen for explosives -- five years after the "shoe bomber" incident of 2001 -- and the agency also deployed federal air marshals overseas.Metal detectors were standard at airports before 9/11, but by March 2010 -- a few months after the "underwear bomber" was apprehended on a Christmas Day flight after a botched mid-air attack using a device hidden beneath his clothing -- full-body scanners were starting to be installed at U.S. airports, and about 500 were in action by the end of that year.By July 2017, in response to increased terrorist interest in hiding improvised explosive devices inside commercial electronics and other carry-on items, the TSA began requiring travelers to place all personal electronics larger than a cell phone in bins for X-ray screening. By the following February, facial recognition technology was also being piloted.Safety on board"It used to be (that getting) into a cockpit on an American aircraft that was flying in American airspace was as easy as the doors you use to get into the (toilet)," O'Keefe recalled.Bulletproof and locked cockpits became standard on commercial passenger aircraft within two years of 9/11.The Arming Pilots Against Terrorism Act was signed into law in November 2002, and by the following April, the first weapon-carrying pilots were on board U.S. commercial flights.While aviation fans and children could once hope to get a visit to the flight deck, that dream swiftly came to an end.Private jet pilot and social media star Raymon Cohen told CNN Travel in July that he believes the unprecedented inaccessibility added to flying's mystique."People are not welcome in the cockpit anymore, so it's like a big secret," Cohen said. "Now this (following pilots on Instagram) is one of the only ways people can see what's happening."Passenger confidenceThe immediate impact of 9/11 included a big drop in travel demand. Not only had passenger confidence taken a hit, but the additional security meant the flying experience was no longer fast and hassle-free.In 2006, the International Air Transport Association (IATA) estimated that airline revenues from domestic U.S. flights fell by $10 billion a year between 2001 and 2006. For comparison, the net losses globally due to the COVID-19 pandemic in 2020 were $126.4 billion in total, according to the IATA.In a study from 2005 on the impact of 9/11 on road fatalities, Cornell University's Garrick Blalock, Vrinda Kadiyali and Daniel H. Simon found an increase in travelers choosing to drive rather than fly. The unintended consequence of this was that "driving fatalities increased significantly following the terrorist attacks of September 11, 2001." They estimated that a total of 1,200 additional driving deaths in the past five years were attributable to the effect of 9/11.Speaking to CNN ahead of the 20th anniversary of 9/11, Kadiyali said, "There's been the fall of Kabul and all these recent events in Afghanistan (...) It did cross my mind whether people would start getting nervous about flying again."Delays, long lines and confusion over restrictions are also all back on the agenda in the pandemic era.As to whether something like 9/11 could happen again, O'Keefe reflected upon the fact that the greatest achievements of Homeland Security, and of security services around the world, can never be shared with the general public."In the process of educating the public, what you also do is educate the terrorists," so we will never know of all the near-misses, he said. "You almost get into a false sense of security."That September morning in 2001 "flipped the switch right away from almost non-existent security to unbelievable, in-your-face, all the time."However, two decades later, there have been no aviation-based terrorist attacks anywhere near the scale of 9/11. Said O'Keefe, "These security measures have worked."