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 September 11, 2001, a team of 19 hijackers was able to board four different domestic flights in the northeastern US 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 US 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 September 12 and has never gone back to what it was on September 10.”

The US 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 US 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 September 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 US 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 US — 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 was introduced in some countries.

The creation of the TSA
Airport screening in the US 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 US Department of Homeland Security, which was formed a year later, the TSA took over all the security functions of the FAA and US 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.”

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 terrorists 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 midair attack using a device hidden beneath his clothing — full body scanners were starting to be installed at US 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.