NEW YORK (AP) — Americans remembered 9/11 on Sunday with tear-choked tributes and pleas to “never forget," 21 years after the deadliest terror attack on U.S. soil.

Nikita Shah headed to the ceremony on the ground in a T-shirt that bore the de facto epigraph of the annual commemoration — “never forget” — and the name of her slain father, Jayesh Shah.

The family moved to Houston afterward but has often returned to New York for the anniversary of the attack that killed him and nearly 3,000 other people.

“For us, it was being around people who kind of experienced the same type of grief and the same feelings after 9/11,” said Shah, who was 10 when her father was killed at the World Trade Center on Sept. 11, 2001.

Victims’ relatives and dignitaries also convened at the two other attack sites, the Pentagon and a field in Pennsylvania.

Other communities around the country are marking the day with candlelight vigils, interfaith services and other commemorations. Some Americans are joining in volunteer projects on a day that is federally recognized as both Patriot Day and a National Day of Service and Remembrance.

More than two decades later, Sept. 11 remains a point for reflection on the attack that reconfigured national security policy and spurred a U.S. “war on terror” worldwide. Sunday's observances, which follow a fraught milestone anniversary last year, come little more than a month after a U.S. drone strike killed a key al-Qaida figure who helped plot the 9/11 attacks, Ayman al-Zawahri.

It also stirred — for a time — a sense of national pride and unity for many while subjecting Muslim Americans to years of suspicion and bigotry and engendering debate over the balance between safety and civil liberties. In ways both subtle and plain, the aftermath of 9/11 ripples through American politics and public life to this day.

And the attacks have cast a long shadow on the personal lives of thousands of people who survived, responded or lost loved ones, friends and colleagues.

Firefighter Jimmy Riches’ namesake nephew wasn’t born yet when his uncle died, but the boy took the podium to pay tribute to him.

“You’re always in my heart. And I know you are watching over me,” he said after reading a portion of the victims’ names.

More than 70 of Sekou Siby's co-workers perished at Windows on the World, the restaurant atop the trade center's north tower. Siby had been scheduled to work that morning until another cook asked him to switch shifts.

Siby never took a restaurant job again; it would have brought back too many memories. The Ivorian immigrant wrestled with how to comprehend such horror in a country where he'd come looking for a better life.

He found it difficult to form the type of close, family-like friendships he and his Windows on the World co-workers had shared. It was too painful, he had learned, to become attached to people when “you have no control over what’s going to happen to them next.”

“Every 9/11 is a reminder of what I lost that I can never recover,” says Siby, who is now president and CEO of ROC United. The restaurant workers' advocacy group evolved from a relief center for Windows on the World workers who lost their jobs when the twin towers fell.

On Sunday, President Joe Biden spoke and laid a wreath at the Pentagon. At the same time, first lady Jill Biden spoke in Shanksville, Pennsylvania, where one of the hijacked planes went down after passengers and crew members tried to storm the cockpit as the hijackers headed for Washington. Al-Qaida conspirators had seized control of the jets to use them as passenger-filled missiles.

Vice President Kamala Harris and husband Doug Emhoff joined the observance at the National Sept. 11 Memorial in New York. Still, by tradition, no political figures speak at the ground zero ceremony. It centers instead on victims' relatives reading aloud the names of the dead.

Readers often add personal remarks that form an alloy of American sentiments about Sept. 11 — grief, anger, toughness, appreciation for first responders and the military, appeals to patriotism, hopes for peace, occasional political barbs, and a poignant accounting of the graduations, weddings, births and daily lives that victims have missed.

Some relatives also lament that a nation that came together — to some extent — after the attacks have since splintered apart. So much so that federal law enforcement and intelligence agencies, which were reshaped to focus on international terrorism after 9/11, now see the threat of domestic violent extremism as equally urgent.

A Warren County doctor who responded to Ground Zero 20 years ago said the efforts to find survivors among the wreckage of the World Trade Center have helped shape his career in emergency rooms ever since.

On September 11, 2001, Dr. Randy Mariott got the call to make the trip to New York City as part of Ohio Task Force 1.

“We thought that we were heading there with significant potential of being able to rescue live victims,” he said. “That was our hope.”

For the medical director of Premier Health EMS Center of Excellence, that hope never materialized. When they got to New York City on Sept. 12, there were no survivors to find. The team instead focused on recovering remains, and Marriott attended to the health and safety of other first responders.

“That was the most difficult part,” he said. “The realization that nearly 3,000 of our fellow citizens, not to mention over 400 first responders, were still in that pile and we probably could do nothing to help them.”

His time at Ground Zero has often influenced his work as an emergency department doctor at Premier Health’s different hospitals across the area, he said.

“I think that just the preparation has played a role in my career,” Marriott said. “I’ve tried to focus on doing more with less.”

In addition to having the experience influence his career, he also said the terrorist attacks may have helped make a doctor out of his son.

“I picked him up at his middle school on the way to the hospital to get medical gear that we had to take with us,” Marriott said. “He actually helped me carry out the medical supplies that went to Ground Zero.”

Warning: The above video has footage that may be upsetting to some viewers.

Sept. 11, 2001, was a sunny day in New York City.

Randal Robinson, of Savannah, Georgia, had just arrived for a morning U.S. Customs Service seminar, being held on the fourth floor of Six World Trade Center.

One of the buildings that would soon be gone.

"And then when the thing hit...Boom. One of the New Yorkers said, 'Oh we don't have earthquakes in New York very often.'

And then.... Boom boom rumble rumble.... Dragging scraping sound.

Oh... somebody's moving furniture.

And we thought.... 'Naah.'

Then somebody stuck their head in the door and shouted, 'Everybody out....now! Do not go down the elevator. Come down the steps and follow me.'"

Robinson remembers it all like it was yesterday. How they all dashed down the stairs to the street below. He looked up and saw the smoke.

Somebody said a corporate jet must have struck the building. He couldn't figure out how the pilot could have done it.

"Well heck, it had to be a heart attack or a stroke," he said. "It was beautiful weather. You couldn't miss seeing it. So I said, 'Well, hopefully, they'll get the fire put out soon.' So I pulled out my camera and started taking the pictures."

He snapped about a dozen images before realizing just what he was witnessing.

"So then when I ran out of film," he said, "I put in another roll of film and started to take more pictures and then someone said, 'Oh my gosh, people are jumping out.' So I said no more pictures."

He thought the firefighters would put it out. But they had no chance.

"In a few minutes, someone said 'Oh my god, here comes another one.' This other jetliner was coming down the river. Banked hard left. Came over our heads, engines screaming and hit Two World Trade Center."

He finally made it back to his hotel room and watched on TV with the other guests as the towers fell, including the building he had evacuated.

"...Thank the Lord we all got out safely."

Two decades after the 9/11 attacks shook America to its core by crashing two planes into the World Trade Center, some of the men and women who were first to respond to the tragedy in lower Manhattan are recalling their harrowing experiences as this somber anniversary approaches.

James Hill, Gerry Giunta and Michael Gomes are members of the Massachusetts Task Force 1. The agency has responded to countless natural disasters since it was first founded in the early 1990s. But September 11, 2001, was the team’s first time dealing with a terrorist attack.

James Hill still remembers pulling into New York City a few hours after the World Trade Center towers had collapsed.

“We turned the corner and there was an airplane jet engine laying on the sidewalk,” Hill recalled.

For Gerry Giunta, it was the color from the building’s ash and debris that still sits in the back of his mind some 20 years later.

“Everything was monochromatic grey and as we got further it was like snow,” he said.

These three men were among the first to arrive in New York City, mere hours after the country was shaken to its core. But as they barreled toward New York City in old military vehicles, with no FM radios or cell phones, no one had any idea what to expect.

"It was really strange. Everything was grey. There was no color to anything; there was dust covering everything. You just had to take a deep swallow knowing what you’re going into and what you’re faced with,” Gerry Giunta added.

For eight days, with no sleep, the task force worked in coordination with New York City authorities, hoping against hope to find someone alive while sifting through what remained of the World Trade Center.

“It was 22 acres of utter destruction. You basically had two buildings, each floor was an acre, and the buildings were 110 stories high, so we were looking at 220 acres of concrete. And yet, we never saw a piece of concrete larger than a grain of sand,” Michael Gomes said about the impact of the towers imploding.

As the days passed, more and more families showed up looking for loved ones. Exhausted, this team continued to work, using the sphere from the World Trade Center, which survived the collapse, as their compass for every mission.

“All of that was a hole in the ground. We’d use that ball like it was a clock,” responder James Hill said.

The years have not been kind to those who were on the ground in those dark days and weeks after the towers fell. An estimated 4,000 first responders have died in the last two decades from illnesses related to 9/11. Two of them were from this task force.

“Out of 72 people, we’ve lost two. It’s something that’s always in the back of your mind, those aren’t great odds,” Michael Gomes said.

This team continues to move forward though, responding to new disasters as they unfold and using lessons learned from 9/11.

But like the memorials, now standing as reminders, the men and women of this task force are taking this 20th anniversary of 9/11 to reflect.

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 [9/11] 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 checks

Some 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 TSA

Airport 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 screening

Some 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 confidence

The 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."