Last year, TSA officers discovered a record-setting 4,239 firearms in the carry-on luggage of passengers across the United States, a 7 percent increase from 2017.

At Pittsburgh International Airport, the numbers have remained steady. TSA officers seized 32 firearms in 2017 and 34 last year. Atlanta’s Hartsfield-Jackson International Airport led the nation in firearm seizures in 2018 with 298, including 32 in a single month.

“The numbers are pretty low and pretty consistent year to year” for nearly a decade, said Inspector William Palmer, who leads the airport division of the Allegheny County police. “We get one every 10 days or so.”

The incidents tend to be similar as well, Palmer said. Typically, a TSA screener spots a firearm in a carry-on bag as it glides through the X-ray machine. A county police officer — one is stationed at each checkpoint 24 hours a day, seven days a week — responds, seizes the weapon and notifies the FBI Civil Aviation Security team.

Because most guns end up at checkpoints unintentionally — at least at PIT — the FBI generally allows county police to conduct the investigation. But how that investigation plays out and whether it results in charges depends on several factors.

First, there’s state law. Some states prohibit firearms in airports; Pennsylvania is not one of them. At PIT, licensed gun owners can be charged under federal law. Every firearm investigation at PIT is sent to the U.S. Attorney’s office for review, but few charges result, Palmer said.

“They consider intent,” he said. “And, in most cases, people just forget it was in there.”

It’s a different story for passengers caught with firearms and no permit, says Palmer. County police can file state charges for carrying a gun without a permit. Three passengers were charged in 2017 and three more in 2018.

Still, anyone caught with a firearm at a TSA checkpoint is subject to a federal fine. First-time offenders can expect to hand over nearly $4,000. Repeat offenders can be fined up to $13,000.

Palmer acknowledged that the fines are steep, even for people who had no ill intent. But he has little sympathy for them as they pay their penalties and navigate the official process.

“It’s the responsibility of gun owners to know where their firearms are at all times,” Palmer said.

Here are the airports that led firearm discoveries in 2018, according to TSA records:

1. Hartsfield-Jackson Atlanta International Airport (ATL): 298
2. Dallas/Fort Worth International Airport (DFW): 219
3. Phoenix Sky Harbor International Airport (PHX): 129
4. Denver International Airport (DEN): 126
5. Orlando International Airport (MCO): 123
6. George Bush Intercontinental Airport (IAH): 117
7. Fort Lauderdale-Hollywood International Airport (FLL): 96
8. Austin-Bergstrom International Airport (AUS): 93
9. Dallas Love Field Airport (DAL): 89
10. Nashville International Airport (BNA): 86