DENVER (KDVR) — As the U.S. gets ready to mark 20 years since the attacks on 9/11, the Transportation Security Administration is giving the public new insight into the lengths it takes at Denver International Airport to keep passengers safe.

In 2021, Denver ranked as the fifth busiest U.S. airport for the TSA with just fewer than 7.4 million passengers from January to June, alone. In 2019, TSA screened around 23 million passengers in Denver, making it the 10th busiest airport that year.

Most passengers already know the drill: check your bag, wait in the security line to show your ticket, scan your backpack, take off your shoes, walk through the X-ray machine and go.

That process became the norm at U.S. airports after 9/11 with the creation of the TSA, which standardized security procedures across the country to ensure a high level of safety.

“Our main thing is we don’t want that to happen again — 9/11,” TSA manager at Denver International Airport Doug Johnstone told FOX31.

How TSA screens airport visitors

Their job begins before anyone even boards a flight.

TSA’s Secure Flight program checks the name of each passenger against the Terrorist Watch List and No Fly List before boarding passes are issued. This has been done for 100% of domestic and foreign flights flying into or out of the U.S. since 2010. TSA conducts the same type of screening for foreign airlines with flight paths that cross over the United States.

“Individuals known to pose a threat to aviation are not issued a boarding pass and are not allowed to fly — period,” TSA spokesperson Lorie Dankers said.

Anyone without a boarding pass is no longer allowed past the first TSA checkpoint officer.

Once inside the security line, passengers undergo routine screening of themselves, their clothing and their hand luggage. All 23 million annual passengers flying out of DIA will undergo checkpoint screening.

“This is the main screening to get you to your plane and your destination,” TSA lead officer Carley Krenz said.

Krenz is also trained as a Passenger Support Specialist, which helps passengers who may need additional help or alternate screening due to a medical condition. However, PSS officers are also on the lookout for passengers trying to find a loophole in security.

“They must be aware of attempts of ‘social engineering’ by travelers who want to circumvent the screening process by claiming a special circumstance prevents them from completing the screening process,” Dankers said.

PSS officers typically help passengers traveling with infants, breast milk, diabetic supplies, wheelchairs and other atypical travel items.

“We trust the passenger. We just want to verify that them and their property is still safe,” Krenz said.

Assistance from a PSS officer can be scheduled in advance or on the spot at any U.S. airport.

What happens with your bags at the airport

While passengers undergo screening, their checked bags are put through a screening process of their own. By law, all baggage that travels on a commercial aircraft must be screened.

“The traveling public doesn’t see the baggage operation and it is just as important as the screening that is going on in the checkpoint,” Johnstone said. “We screen all the bags. Every single bag that gets checked.”

In Denver, there are 10 miles of conveyer belts coordinating a highway of luggage through the airport. On Sunday, Aug. 1, TSA scanned about 72,000 passengers and 41,801 bags.

“The main thing we’re looking for here with checked baggage is explosives, IEDs,” Johnstone said.

TSA screens checked luggage using high-tech machinery that creates a 3D X-ray image of the contents.

“That technology on the CTX is actually looking for hundreds of different items in that bag,” Johnstone said.

TSA said 95% of bags make it through screening in about five to seven minutes. If the scanner detects any questionable materials or flags a bag for another reason, it is sent for a follow-up screening involving a TSA officer inspecting the image. Some bags also require an additional physical inspection when the image is still too vague.

“Our main job is security driven: making sure all bags get on that plane without any security threat in there,” Johnstone said.

But that’s not all: unannounced screenings happen airport-wide

While passengers can expect TSA to screen all passengers and bags every time they fly, there is another layer of airport security that is designed to be unpredictable.

“We do unpredictable continuous screening,” TSA officer Jennifer Lackey said.

Lackey and her team of specialized screening officers do unannounced screenings anywhere and everywhere in the airport including checkpoints, terminals, concourses and ramps to deter insider threats from TSA officers, airport employees, pilots and other airline staff.

Screenings include identification checks and bag searches similar to what passengers experience.

“It’s just to ensure we cover all the bases so that zero threats will make it through,” Lackey said.

Also, there are dogs

Another, more well-known tool designed to be unpredictable is TSA’s Passenger Screening Canines. Dogs like Beli, a German shorthair pointer who works at DIA, are specially trained to sniff out explosives and explosive residue inside the airport, on airplanes and on people.

“PSCs are trained to navigate among large groups of people to pinpoint the source of an explosive odor. PSC handlers are trained to read the dog’s behavior when it indicates an explosive scent has been detected often without the source being aware and even if the source is mobile,” Dankers said.

Dogs have been used as a tool to detect explosives for many years, including prior to 9/11. However, in the wake of the tragic attacks, canines are now trained to focus on passengers, too.

“That was a new addition to the program post-9/11,” TSA canine supervisor Casey Stevens said. “So we focused on the person as well, and that’s not something that was commonplace prior to 9/11.”

It is unknown how many dogs like Beli work at DIA and other airports across the country. Their schedules are kept confidential as part of their unpredictability.

“It’s a huge psychological deterrent,” Stevens said.

According to Stevens, criminals know they will be unable to get explosives past a dog, which means fewer try to do so.

“I think it’s a really calming experience for a lot of the traveling public to see the canine officers out and about working in these various areas,” Stevens said.

And of course, some TSA security measures aren’t made public

Still, there are even more layers to TSA’s security process the public does not know about that are designed to keep passengers as safe as possible. TSA says 9/11 is its constant reminder and motivation to make each day safer than the last.

“As technology advances we also advance,” Krenz said, “and we’re very agile and always changing the way that we screen because the threat is ever-evolving.”