WASHINGTON, D.C. — Year after year, more Americans attempt to board planes with concealed firearms in their carry-ons. They also come with hidden swords, hatchets, sharpened ninja stars and even gunpowder.

Invariably, the response from passengers when officers from the Transportation Security Administration seize the weapons is: Oops, I forgot I had it.

“It’s always astonishing to me that people can forget they have a weapon in their carry-on,” said TSA Administrator Peter V. Neffenger. “I’m not sure why people continue to do this.”

Not everyone buys the excuse of forgetfulness.

“They didn’t forget their pants. It’s beyond me,” said David Borer, general counsel of the American Federation of Government Employees, a union that represents the nation’s 42,000 or so transportation security officers.

Whether the reason is memory lapse or a desire to be prepared should armed terrorists once again try to commandeer an aircraft, the seizure of a record 2,653 firearms last year at airport checkpoints is but one aspect of an evolving security panorama as the nation passes the 15-year anniversary of the Sept. 11 attacks.

The security ritual now has a familiar rhythm: Shoes off. Laptops out. Everything through the X-ray machine. Nearly 2 million passengers endure the drill each day. Many hate it. A few get unruly.

The tools deployed by the security agents — including full-body scanners — get ever more sophisticated while the most basic of questions go without a satisfying answer: Do all the security measures work? Are terrorists truly deterred? Are we safe?

Experts agree on only one thing. Heightened airport security is here to stay.

“It’s going to take a long time before we stop taking our shoes off,” said Bruce Schneier, a security technologist and fierce critic of the TSA.

It’s hard to pick apart the security procedures the federal government has adopted and not arrive at the conclusion, as Schneier has, that much of it is “security theater.”

Seeming failures abound. An audit last year found that TSA officers found weapons only three times when undercover investigators passed through airport security checkpoints 70 times with weapons or mock explosives, a failure rate of 95 percent. The then-administrator lost his job.

“We are not safer than before 9/11, regardless of the money and energies spent to change airport security,” said Michael Boyd, an aviation consultant and longtime former airline executive based out of Evergreen, Colo. “The TSA approach is a dud. It is a giant bureaucracy with zero accountability for failure.”

Events in the past month underscore how TSA officers, who are unarmed, behave in the face of potential terror. On the night of Aug. 14, when false reports circulated of gunshots at John F. Kennedy International Airport in New York, TSA officers and civilian security guards left their posts and joined a stampede of hundreds of travelers. Borer said TSA policy was for officers to “run, hide, fight,” in that order, in the event of any attack, and that the officers had not been disobeying orders. Still, the incident raised questions about what might unfold in the event of a real terrorist attack.

Two weeks later, panic broke out at Los Angeles International Airport when loud noises led to rumors of an active shooter. Several terminals were evacuated, and passengers and TSA officers alike breached security doors to flee to the airport tarmac.

Panicked people, Boyd said, were “chasing off in all directions like a herd of gazelles running from thunder. TSA has no plan in the event of an incident, except to tell people to run away from the noise, or dump them into the street in a nice tight crowd for a terrorist target.”

The hassles of slipping off shoes, pulling laptops out of bags and emptying coins from pockets have spurred applications for expedited security screening. Known as TSA PreCheck, the program lets low-risk travelers ease through checkpoints without removing shoes.

Some 12,000 applicants a day pony up the $85 for five-year memberships, after waiting six weeks for appointments, and total numbers have surpassed 3.5 million. According to Secretary Jeh Johnson of the Department of Homeland Security, 96 percent of PreCheck passengers spend an average of five minutes or less at security checkpoints.

“We’ve turned security into have and have-not,” Schneier said: the PreCheck passengers who’ve paid their money and the rest of the traveling public.

Rudeness and the occasional too-intimate pat-down have led to combative encounters.

“There’s just this sort of general hatred of TSA, and some people go off,” said Borer, the union general counsel. “All this scorn gets heaped on them. It’s all the people coming through and saying, ‘Screw you. Don’t touch me.’ It’s awful.”

Passenger anger has occasionally flared. In 2013, an unemployed motorcycle mechanic killed a TSA officer and wounded two other officers and a teacher in a rampage at Los Angeles International Airport. A notebook he left behind referred to the TSA’s “Nazi checkpoints.”

In 2015 in New Orleans, a taxi driver attacked TSA officers with a machete and wasp spray, injuring one of them, before being shot by a sheriff’s lieutenant. The assailant later died.

Rating the effectiveness of security procedures is a divisive endeavor. Experts disagree.

“The two things that have improved security since 9/11 — and there are only two — is one, reinforcing the cockpit doors, and two, teaching passengers that they have to fight back,” Schneier said.

Some airline pilots, protected by the reinforced locked cockpit doors, now maintain handguns at arm’s reach in case of intruders.

Awareness among passengers of their own potential roles in thwarting terrorism soared after Sept. 11, 2001, when courageous passengers aboard United Flight 93, bound from Newark, N.J., to San Francisco, took on the four hijackers. The passengers and crew tried to regain control of the flight, leading to its crash in a field in Pennsylvania but preventing the hijackers from slamming the airliner into a still-unknown target, perhaps the White House or the U.S. Capitol.

Fighting back now seems ingrained in some passengers.

“Go to an airport and pick 10 random people, and they’ll tell you, ‘We know we have to do this, 100 percent,’” Schneier said.

Indeed, passengers have averted several terrorist incidents. Richard Reid, a Brit whom al-Qaida had recruited to board a Paris-Miami flight with explosives in the soles of his shoes in late 2001, was tackled by passengers and crew members before he could ignite the explosives.

Travelers also subdued a Nigerian man, Umar Farouk Abdulmutallab, after he attempted to detonate plastic explosives hidden in his underwear on a Christmas Day flight from Amsterdam to Detroit in 2009.

“Ultimately, it’s going to be fellow passengers and alert citizenry who create a better defense for Americans than a government monopoly like the TSA,” said Chris Edwards, an analyst at the Cato Institute, a research center that advocates for minimal government intrusion.

Even the famed air marshals, whose numbers soared after Sept. 11, have come under fire. The program, which puts armed agents on high-risk domestic flights, costs more than $820 million annually. In theory, the marshals are the last line of defense before a terrorist hijacking. In practice, more air marshals appear to have been arrested for felony crimes they themselves have committed than make arrests in the line of duty.

When the bureau responded to a freedom of information request earlier this year by ProPublica, a nonprofit investigative newsroom, more than seven years after the request was made, it acknowledged that air marshals had been arrested 148 times from 2002 through the early 2012 for various crimes unrelated to their work.

While air marshals attended to “thousands” of medical emergencies and non-terrorist incidents involving unruly passengers, they apparently carry out few arrests of real terror suspects. A Federal Air Marshal Service spokesman, Thomas H. Kelly, did not address a request for a breakdown of incidents.

A Tennessee Republican U.S. lawmaker, John J. Duncan, said in 2010 that the air marshals service had made an average of 4.2 arrests per year from 2001 to 2010, adding that “we are spending $200 million per arrest.”

Even with the travails of the TSA and the air marshals, one fact since Sept. 11 can gladden the heart of any traveler.

“There haven’t been any (successful) terror attacks since 9/11, knock on wood,” said Borer.

That may make passengers feel better, but not experts who see shortcomings.

Some foresee airports with concentric rings in the approaches to terminals, with facial recognition software in active use, pushing a security perimeter outside terminal buildings.

“Our back doors are wide open at airports,” Boyd said. “Ground security for airliners is really weak: things like catering carts, cargo pods, et cetera, have no security.”

“Certain airports present a better ideological target for terrorists: JFK, LaGuardia, LAX, San Francisco, Atlanta,” said Anthony C. Roman, a former pilot who is a security consultant. “We have to be hyper aware — not frightened, but not with our heads in the clouds.”

“Should there be a successful attack on a U.S. airport, I think we’re going to see more intense security procedures,” he said.