The number of guns seized at BWI Thurgood Marshall Airport is climbing sharply, authorities say, mirroring a years-long increase at airports throughout the region and across the country.

Seizures at BWI rose 50 percent in 2016, and are on pace to climb another 33 percent this year. Nationwide, they increased last year by nearly 28 percent.

Officials with the federal Transportation Security Administration, which staffs the security screening areas at BWI and other airports, say they don’t know why seizures are rising.

“It’s a trend that’s very concerning,” TSA spokeswoman Lisa Farbstein said. “It’s a trend we’d like to see go in the opposite direction.”

The numbers remain small: TSA agents confiscated 24 guns at BWI in 2016. But they have increased four straight years, outpacing the growth in air passengers through the region’s busiest airport; they’re matched by similar increases at Washington Dulles International Airport, Ronald Reagan Washington National Airport and airports nationwide; and they’re up again this year.

Nationwide, TSA agents confiscated 3,391 firearms — an average of more than nine per day — in 2016, more than double the number seized in 2012.

Federal law allows air passengers to travel with unloaded firearms only in checked baggage. They must be packed, unloaded, into a locked, hard-sided container. Guns and ammunition must be declared at the ticket counter when the baggage is checked.

Passengers are prohibited from carrying guns onto aircraft. Most of the firearms seized at BWI this year have been small handguns left in purses or other carry-on luggage.

Farbstein said the reasons for the increases are unclear.

The 5.5 percent growth in passengers through BWI last year doesn’t explain the 50 percent increase in guns confiscated. Nationally, passengers were up 6.2 percent; seizures rose more than four times as much.

The number of applications to purchase regulated firearms in Maryland increased 39 percent in 2016, according to the Maryland State Police. That jump could be due in part to the automation of the application process, spokesman Greg Shipley said.

The most common excuses given for being found with a gun, Farbstein said, are that the passenger forgot he or she was carrying it, or that someone else — typically, a spouse or partner — packed the bag.

But once the passenger walks through the security line — past notices that list prohibited items — and puts the bag into the X-ray machine, he or she has illegally brought a firearm onto federal property. “Neither of those excuses fly,” Farbstein said.

One Tuesday morning in April, a 61-year-old Pennsylvania man put his laptop bag on the conveyer belt and watched it disappear through the flap. When a TSA agent stopped him, he said, he realized with horror that he had left his .32-caliber Beretta pistol inside.

“I was scared to death,” he said.

The man agreed to share his account of the situation on the condition that his name not be published.

He said he has a concealed carry permit in Pennsylvania. He said he had taken the handgun out of his truck as a precaution the night before, so that it wouldn’t be stolen if someone broke into the vehicle. He put it in his laptop bag, where he doesn’t normally keep it, he said, and forgot to take it out before going to the airport.

“This happened because I thought it was unsafe to leave my gun in my vehicle,” he said. “It happened out of caution.”

He was arrested, but he said the officer was sympathetic. “‘This happens a lot more than you’d think’ — the officer told me that,” he said.

The man, who said he had never been arrested previously, said his self-esteem crumbled. “It took a month or two for me to start feeling good about myself,” he said.

“You’re not happy with yourself, that’s for sure,” agreed a 62-year-old Delaware man who was found with a gun at BWI in November. He also spoke on condition of anonymity.

“No one in their right mind is going to walk into an airport with a handgun,” he said. “You do it because you don't know it’s there.”

The man was preparing to fly to Calgary, Alberta, to visit a friend who had been diagnosed with cancer. He said he overlooked the handgun while packing his bag.

When a cadre of police showed up at the security gate, he realized what he had done.

“It was just an oversight,” he said. “It was an unusually stressful situation, with not a lot of sleep.”

The man said he was handcuffed and put in a holding cell, where he spent most of the day before being taken to a courthouse in Annapolis. Once he was released, he took an Uber back to the airport, found no more flights available that day, and headed home.

When a TSA agent sees a gun cross the computer monitor, the agent stops the conveyor belt and calls a supervisor to confirm it. Then they contact Maryland Transportation Authority Police. The agents leave the luggage in the X-ray machine until an MDTA officer arrives to confiscate it and place the passenger under arrest.

Passengers caught with guns at BWI are handcuffed and taken to a holding area while the arresting officer fills out a statement of probable cause, MDTA Police Lt. Kevin Ayd said. They are taken for an initial appearance before an Anne Arundel County court commissioner, who decides whether to set bail or release the passenger on his or her own recognizance.

Ayd’s advice: “Make sure you follow the proper procedures so you don’t have to go through an arrest.”

The fine for a first offense is typically $3,000, Farbstein said, but it can be more, depending on the circumstances. The costs of retaining an attorney and rescheduling the missed flight can add up quickly.

“A mistake like this can be very expensive,” she said.

Transporting a gun legally is one of four curriculum areas of a four-hour, State Police-certified firearm training course required for a permit to purchase a handgun in Maryland.

John Finnessy, owner of Fortress Training in Ellicott City, said he begins that part of his course by holding up a firearm and addressing his students sternly.

“Let me just be clear: This does not go in your carry-on bag,” he says, “or you are in trouble.”

Jade Macharia said she was unnerved by the number of guns being caught at BWI’s security checkpoints.

The 25-year-old Towson woman, who was at the airport recently preparing to board a flight to Houston, said the excuses seemed flimsy.

“How do you forget a gun is in your bag?” she asked. “If you’re the type of person who’s going to forget a gun is in your bag, maybe you shouldn’t own a gun.”

Finnessy said he understands how it happens. For gun owners who carry their weapon regularly, bringing it with them becomes a routine part of leaving the house — so forgetting to leave it at home can feel akin to carrying keys accidentally through a metal detector, Finnessy said.

“I would imagine for a majority of those people, it was an honest mistake,” he said. “But you’re dealing with TSA, and you have to abide by their rules.”

Briana Johnson said she understood gun owners wanting another level of protection while traveling. But the 20-year-old Port Deposit woman said she doubted violators simply forgot about their guns.

“You don’t forget your firearm,” she said. “It’s easy enough to check it.”

Johnson, who graduated from Liberty University with an accounting degree in May, was scheduled to fly to Atlanta with her mother this month to visit her boyfriend.

Angela Johnson, 39 echoed her daughter’s skepticism.

“I don’t know how people would not know,” she said.

Briana Johnson said people should have faith in the TSA and law enforcement stationed at the airport to protect them.

“They do their jobs well,” she said.

Pete and Janet Donahoe of San Antonio, Texas, flew to BWI Marshall to visit friends in York, Pa., over the Fourth of July.

They said the increasing number of guns found at security checkpoints didn’t bother them — as long as they were seized and not allowed on the plane.

“I’m glad they found 16,” said Janet Donahoe, 70. “I wonder how many got past them.”

The prohibition of guns in carry-on luggage ought to be basic knowledge for gun owners, said Pete Donahoe, 74. Forgetting about the weapon betrays a carelessness on the owner’s part, he said.

“That’s no excuse,” he said. “Even if it’s true, it’s no excuse. If you own a firearm, you’ve got to take care of it.”