Despite mounting concerns and questions about whether Sol Pais illegally bought a shotgun in Colorado, the Bureau of Alcohol, Tobacco, Firearms and Explosives continues to uphold its stance that the purchase was legal.

Critics of the sale point to a federal regulation that says purchases have to comply with laws of the state where the gun is bought and the state where the person resides. That means Pais, who was not old enough to buy a shotgun in her home state of Florida, should not have been able to buy a shotgun in Colorado.

But others argue laws around gun sales aren’t so clear cut.

Federal laws allow licensed dealers to sell rifles and shotguns to out-of-state residents above the age of 18. But Florida law requires that its residents be at least 21 to purchase any firearm, a law that changed in 2018 in response to the Parkland school shooting. Florida also implemented a three-day waiting period, but Colorado does not have that requirement.

The ATF in Denver repeatedly has said the sale was legal, but has refused to explain how its agents reached that conclusion.

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Pais, 18, was reportedly infatuated with the Columbine massacre and made threats that the FBI deemed credible. She flew to Colorado on Monday and bought a pump-action shotgun and ammunition at a gun store near Littleton shortly after her arrival. After a nearly two-day manhunt, authorities found her dead by suicide in the forest at Mount Evans.

Gun sellers are the ones who are responsible for understanding laws in all 50 states, according to the Firearms Transaction Record, the form dealers and buyers are required to fill out before a gun sale is complete.

That’s why some Colorado gun shop owners opt not to sell firearms to out-of-state residents — each state has its own restrictions and laws, and those laws can change.

Tall Guns LLC in Loveland is one gun shop and training center that only sells guns to Colorado residents unless someone is in the military and can show their orders.

Buddy Meyers, the store’s managing partner, said it’s too complex to keep up with state and municipal law changes across the country.

“From a business perspective, I’m just not comfortable with transferring a firearm to someone with an out-of-state license,” Meyers said. “I feel it’s a responsibility I’ve got to the community.”

Meyers believes the responsibility to know the various laws lies with both the seller and with the CBI and FBI background check systems.

“If there’s a solution, I would think that this needs to be within the CBI system,” he said. “They’re wanting to do these background checks, so then they should know based on acquiring a state driver’s license that a state has a restriction.”

Bighorn Firearms in Denver doesn’t have a blanket policy against sales to out-of-state residents, but owner Ryan Resch recognizes that state laws are sometimes written in ways that can be inconclusive. So, he relies on licensed dealers in other states to shed some light on those laws when he gets an out-of-state customer.

Still, Resch is careful with every customer who walks into his store wanting to buy a gun. He said anything that stands out as something to be concerned about, he and his employees take that into account when selling that person a firearm.

Pais bought the gun from Colorado Gun Broker near Littleton. The owner, Josh Rayburn, posted to Facebook on Wednesday, saying the gun purchase was legal. His post said she was approved through the sale by the required background checks and that she did not say or do anything that indicated she was a threat to herself or others.

Multiple attempts to reach Rayburn on Thursday were unsuccessful.

Gun control advocates criticized the sale to Pais, saying it was irresponsible and illegal.

Lindsay Nichols, federal policy director for the Giffords Law Center to Prevent Gun Violence, said in a statement that gun laws need to be strengthened, especially because more young people are getting guns that shouldn’t be accessible to them.

“Gun dealers have an important responsibility as the main gatekeepers in determining who can and can’t get their hands on a firearm,” she said in an email to The Denver Post. “It’s up to both state and federal law enforcement officials to ensure that these dealers are fully aware of what’s required for each sale.”

Everytown for Gun Safety issued a news release to call the sale illegal.

“Under federal law, a licensed gun dealer such as Colorado Gun Broker may only sell ‘long guns’ such as rifles and shotguns to out-of-state residents if the sale would have been legal in the purchaser’s own state (handguns may only be sold to in-state residents),” the news release stated.

The U.S. Attorney’s Office in Colorado was evaluating potential criminal charges in connection with the Pais incident, including whether there were violations of the federal firearms license regulations. But the office dropped the discussion after Pais was found dead, said spokesman Jeffrey Dorschner.

The ATF determined the sale was legal, he said.

ATF spokeswoman Mary Markos reiterated Thursday that the agency believes the 18-year-old bought the gun legally. She would not elaborate on how ATF officials reached that conclusion.

The Colorado Bureau of Investigation, which runs the background checks on gun buyers, came to the conclusion that Pais could legally purchase a shotgun, said spokeswoman Susan Medina. CBI looks at federal and state databases for any prohibitors that would stop a person from buying a firearm.

Medina referred further questions to the ATF.

The Florida Attorney General’s Office declined to comment, and the Colorado Attorney General’s Office did not return a request for comment.

Some gun rights activists also said the sale appeared to be illegal, based on federal law.

Eric Friday, a lawyer for Florida Carry, which advocates for the right to bear arms, said Florida is one of the few states where laws on long gun sales are different than federal law. And that could be a source of confusion in other states.

So, if a licensee has been in business for a while and has sold guns to people from Florida, he or she might not know about the law change and a background check could still clear the sale, he said.

The InstaCheck background system should be catching those restrictions across state lines, said Chris Stone, director of political operations for the National Association for Gun Rights.

Buyers submit their state IDs, so restrictions should show up for that state, he said.

“The gun store here probably did follow procedure correctly,” he said. “But CBI likely didn’t update its system.”