When Clear Secure went public in June, the offering could have been viewed as a moment of rebirth for a travel industry battered by COVID-19. The New York City–based company provides biometric security clearance at more than 50 airports as well as other screening services at sports arenas, office complexes, and even the New York Stock Exchange where shares of the business are now traded. Following its IPO, Clear is valued at $6.8 billion with $215.3 million in annual revenue. Not bad for a business highly exposed to the struggling commercial airline industry.

The fact that Clear Secure exists at all is a testament to how air travel has proved resilient—and innovative—in the two decades since 9/11. This is especially noteworthy given that Clear’s most serious competitor is the federal Transportation Security Administration, itself a byproduct of 9/11 policymaking. Long the focus of frequent fliers’ scorn, the TSA has proved to be something of a rarity: a vast, $7.78 billion government bureaucracy that has gotten more efficient over time, while also creating a favorable funding environment for private sector innovation.

“Prior to 9/11, the U.S. didn’t have money invested in aviation security,” said Jeffrey C. Price, an aviation security expert, professor at the Metropolitan State University of Denver, and coauthor of the book Practical Aviation Security. “But after 9/11, because of all the government money that funneled through the TSA and its centers of innovation, you saw people pitching different types of technology that changed aviation security—and we now see that innovation being used in all different kinds of industries.”

The notion of securing air travel dates back to the 1950s and early 1960s, when hijackings of commercial aircraft first became a threat, though in those days it was really more of a nuisance than anything else. The typical hijacker back then was “a guy who wanted a bag of money and a quick trip to Cuba or Mexico, anyplace where he wouldn’t face extradition,” Price said.

In response, the government launched the Federal Air Marshal program, but it wasn’t particularly aggressive about strengthening airport security. A hijacking was still a reasonably easy thing to get away with. Then in 1968, the pace of hijackings suddenly intensified, with an incident occurring nearly once every week in the U.S. The most headline-grabbing episodes involved aircraft and passengers kidnapped for multiple days and sometimes flown off to far-flung places like Argentina or Algeria. Though the government enhanced criminal penalties, it was still slow to intervene, and the crisis dragged on through the early 1970s.

The problem worsened yet again as terrorists began to realize that airplanes and airports represented a soft target. As the threat metastasized with national security implications, increased air passenger screening—including metal detectors and scanning of carry-on baggage—became standard in the U.S., with the airlines footing the bill.

For a time, the new procedures worked well enough, though there were holes in the system, especially internationally. These were exploited by a series of hijackings and attacks on airports in Rome and Vienna in 1985, and the bombing of Pan Am Flight 103 over Lockerbie, Scotland, in 1988. A heightened emphasis on airline and airport security emerged anew, and for the next decade, incidents involving U.S. aircraft were rare. Then came 9/11.

In the aftermath of the terrorist attack, the industry came to realize that aviation security had been an afterthought for far too long, with tragic consequences. “The airlines look at things from a cost-benefit analysis perspective, and you can understand how, throughout the 1990s, they may have asked themselves: ‘Why should we pay more for security for something that hasn’t happened in years?’” Price said.

It was immediately evident that the airline workers staffing checkpoints tended to be poorly paid, meaning there was high turnover, as much as 100% a year in some facilities, according to Price. Vetting of these transient workers, in terms of criminal background checks and the like, was spotty. The tools were also outdated: The X-ray machines employed at security checkpoints were rudimentary, and configured in such a way that they were better at detecting whether a bag was oversize rather than if any problem items were packed inside. Meanwhile, checked luggage on passenger flights wasn’t screened at all. Neither was commercial air cargo.

The TSA was legislated into existence in November 2001 and, for the first few years, it took a heavy hand in correcting for these deficiencies. Flying became something burdensome. Long screening lines snaked through the public spaces in airports. Though they failed to cause harm, the shoe bomber and the underwear bomber prompted new levels of scrutiny of passengers. Behind the scenes, new TSA personnel and equipment filled terminals. Airport managers suddenly faced a major loss in leasable (and therefore potentially revenue-generating) square footage, throwing out of whack the economics of operating an airport.

But if the flying public and airport staff found the TSA to be overbearing, technology suppliers found a willing customer. To create better imaging systems, for example, the TSA spent more than $2 billion with companies such as Smiths Detection, L3, and Rapiscan Systems. The result: Those once controversial, now standard full-body scanners were near universally deployed by 2016. In the meantime, the TSA became increasingly interested in biometric technology, which uses fingerprints, retina scans, and facial recognition technology to validate the identity of travelers who have been prescreened for security.

Clear was originally launched in 2003 by Steven Brill, the American Lawyer founder and entrepreneurial journalist who wrote a book titled After: The Rebuilding and Defending of America in the September 12 Era. Eyeing growth, the company, then known as Verified Identity Pass Inc., announced that it had raised more than $44 million in August 2008 from a group that included Lehman Brothers and Brill himself. But then the financial crisis intensified (Lehman filed for bankruptcy less than a month later), and the company faltered, ultimately ceasing operations in 2009.

Hedge fund managers Caryn Seidman-Becker and Ken Cornick acquired Clear’s name and assets out of bankruptcy the following year, and relaunched the company. In the decade since, the business has moved beyond offering a souped-up background check (TSA PreCheck does the trick now) to creating what Seidman-Becker, now the company’s CEO, has described as “a secure identity platform” that uses biometric information to hurry its 7 million members through expedited lines at security checkpoints.

“Anyone in biometrics is in an expansive growth industry,” Price said. “The TSA was headed toward biometrics before the pandemic hit, and now that air travel is coming back, one of the advantages of biometrics is less personal contact. I may not need to touch your boarding pass or your phone if I can verify your identity through some kind of biometric scan. And there are opportunities to use biometrics to tighten security access among airport workers as well.”

The potential business opportunity extends far beyond airports, at least as far as Clear’s investor pitch is concerned. “We think there are enormous opportunities not only in travel on a global basis but to bring the Clear platform to so many different verticals,” Seidman-Beckertold CNBC’s Squawk Box on the day of Clear’s IPO. “And my hope is that, in a few years, aviation is our smallest vertical because the others are so much bigger.”

Since its IPO, Clear has announced a new service to restaurants, in partnership with OpenTable, to preclear that guests are vaccinated against COVID-19 prior to their dinner reservations. It has also provided vaccination credentialing for attendees at New York Fashion Week, and even for visitors to the 9/11 Memorial and Museum in lower Manhattan. Competitors like Aware, Daon and Yoti are working on new applications for biometric technology, too.

The most remarkable technological change, then, may not be how airport security adapted in the aftermath of 9/11, but rather how those same security measures will now inevitably serve as a template for how we get back to our “normal” lives in the aftermath of COVID-19.