New York (CNN) Last year, thousands of people trying to buy guns from dealers made false statements about their past on the required federal form, then went on to fail the background check due to a serious criminal record or other disqualifier. Lying to a licensed gun dealer is a felony punishable by a fine or up to 10 years in prison.

Breaking that law might seem risky. But in September, a Government Accountability Office study made it clear why those who "lie and try" would take their chances. Regional offices at the Bureau of Alcohol, Tobacco, Firearms and Explosives received 12,710 cases of firearm background check denials for further investigation in fiscal year 2017, the GAO found, but the government prosecuted only 12 people. More than 99.9% of those who were investigated escaped with nothing more than a warning.

Past and present ATF agents and prosecutors told CNN that, given limited resources, they're not inclined to prioritize the nonviolent crime of lying on a form over more serious charges, like gun trafficking. The very few "lie and try" cases they prosecute focus on dangerous "trigger pullers" who clearly know they're prohibited from owning a firearm -- career criminals with a history of violence or activity in the drug trade.

But a 2006 internal ATF briefing paper obtained by CNN suggests that gun form liars are far more likely to go on to commit a gun crime than even many experts recognize. When ATF analyzed firearm denial cases sent to field offices for investigation during a seven-year period, it found that 10%-21% of that group went on to be arrested for a crime involving guns.

Take John Zawahri, who tried to buy a gun at a California dealer in 2011. State investigators spotted at least one disqualifier -- he had been held at the UCLA hospital psychiatric ward as a violence-prone teen. The California Department of Justice denied him the purchase.

Like almost all gun-form liars, Zawahri was not prosecuted. Undeterred, he built his own semi-automatic rifle mostly from mail-order parts and, two years later, went on a rampage. On June 7, 2013, he set his home on fire, executed his father and older brother with shots to the chest, then hijacked a car to Santa Monica College and killed three others. Dozens of students and bystanders managed to survive by hiding behind restaurant tables and library doors. Police officers caught up to Zawahri at the college library, where they shot him 25 times in a gun battle.

Would prosecution for having lied on a gun form have stopped Zawahri from building his own gun and going on to use it? "I don't believe it would make a difference. There would have been some short jail term, or probation, and he still would have accumulated the gun parts in the two years after that," Santa Monica Homicide Detective Chad Goodwin told CNN.

But a surprising range of authorities have argued that enforcing "lie and try" could make a difference -- including not only anti-gun groups and criminal justice academics, but also former law enforcement officers, the Department of Justice under the Trump administration, and even the National Rifle Association. By failing to bring charges in cases like Zawahri's, the government is leaving thousands of lawbreakers without any new blemish to their names.

Like almost all Americans who lie on federal gun forms, John Zawahri was not prosecuted. He was one of many gun form liars who go on to commit crimes with a gun.

That reduces the law's function as a deterrent to prohibited possessors who might hope to get lucky with the federal background check system. It also allows gun form liars to fly under the radar for law enforcement -- and to avoid being assigned a probation officer who would monitor them to make sure they didn't obtain a weapon. And, in the event of a later crime, it leaves what could have been a significant previous charge -- which might typically lead to more prison time -- off their records.

David Kennedy, a professor at John Jay College of Criminal Justice, said that prosecutors and law enforcement officials should rid themselves of the idea that lying on gun forms is a crime not worth pursuing. "It's worthwhile to go after these cases. The prosecution in question here is failing to fill out a form accurately. That doesn't seem like a big deal. But prosecution is a means to an end -- getting dangerous people off the street."

In Zawahri's case, failure to prosecute probably meant that the young man's years-long obsession with guns and violence stayed below the notice of investigators. Instead, his name merely became one of thousands sitting on law enforcement lists of those who have tried to purchase guns illegally and failed.

A low priority for overburdened agents

When you try to buy a firearm at a licensed dealer, you must fill out a federal form that asks if you're currently under indictment, a fugitive from justice, using any illegal drugs, subject to a restraining order, or inside the United States illegally. It also asks if you've previously been convicted of a felony or domestic violence, adjudicated "as a mental defective," dishonorably discharged from the military, or if you've ever renounced US citizenship.

If you're prohibited and answer truthfully, a gun dealer is supposed to scrap the sale. If you lie, a subsequent background check conducted by law enforcement is supposed to find any disqualifiers in your record. Twenty-nine states and Washington DC rely on the FBI for all firearm sale background checks, 13 states do their own, and 8 split the work. Then the cases are referred to law enforcement: It's up to ATF to investigate federal background check denials, while local police handle state-level denials. After the law enforcement agents investigate, they present potential cases to local or federal prosecutors, who ultimately decide whether to charge someone with a crime.

The federal gun form asks if you're currently under indictment, a fugitive from justice, using illegal drugs, or inside the United States illegally, among other things.

The federal form, and the penalty for false statements on it, is there to discourage prohibited possessors from trying to buy guns in the first place. But a person only breaks the law if a prosecutor can show they "knowingly" made a false statement. That can be difficult to prove. Some people who approach a gun shop counter might legitimately forget about a decades-old domestic violence conviction or a felony they committed as a teen, current and former ATF agents said.

Still, even for someone who is perfectly aware they're a prohibited possessor, it can be worth trying to buy a gun, because so few cases meet US attorneys' standards for potential criminal prosecution. The federal background check system blocked 112,090 attempts to buy a gun in fiscal year 2017. About a tenth of them were referred to ATF field offices for further investigation.

Even when ATF agents do discover a potential gun buyer committed a crime, US attorneys usually don't have the appetite to take on these cases. According to the recent GAO report, federal prosecutors rarely pursue criminal charges, due to a perception that trying these cases in court "may offer little value to public safety because the offender does not obtain the firearm."

States that handle these cases are also reluctant to divert troopers or local police away from their regular duties to look into this crime. When the GAO surveyed the 13 states that do their own background checks for all gun purchases, it found that 10 of them don't even investigate these denials, "citing competing resource demands" and a lack of laws on the books. The only states that investigated gun purchase denials were Oregon, Pennsylvania and Virginia -- and cases in those states had a high bar.

"We were surprised about the challenges," said Gretta Goodwin, who oversaw the study as GAO director of homeland security and justice issues. "States were pretty clear with us that doing these investigations could be disruptive. They were taking state troopers off the road. They were having to make these kinds of sacrifices or tradeoffs."

To some criminal justice experts, the shortage of law enforcement resources to crack down on illegal attempted purchases of firearms is a symptom of a larger problem: We live in a country with more guns than we can afford to track. The sheer number of guns in civilian hands, they say -- 393 million, by one count -- fuel this problem in a way that will always outpace police efforts.

"This resource strain is a function of the system we have in place. There's a huge social cost for having a Second Amendment and relatively lax gun laws," said Daniel Webster, a professor at Johns Hopkins Bloomberg School of Public Health.

A stunning internal report

In an interview, former ATF special agent in charge David Chipman confirmed that he commissioned the 2006 internal report. He had been "absolutely stunned" by the results, he said.

"The assumption was these people weren't a threat," Chipman told CNN. He is now a senior policy adviser at the pro-gun control group Giffords.

The headquarters of ATF in Washington, DC. Internal research there found that a significant percentage of those left unprosecuted for the felony of lying on a gun form will go on to commit a gun crime.

The 30-page internal study explored how, despite an order from then-Attorney General John Ashcroft to crack down on "those who attempt to purchase guns illegally," ATF agents rarely presented cases to prosecutors -- even if the cases met these attorneys' stringent guidelines. The study also highlighted the surprising importance of these cases by taking a closer look at some of the 82,047 cases ATF sent to its field offices for investigation from late 1998 until early 2006. Out of a random sample of 202 cases, 30 of them showed a "subsequent arrest history (that) appeared to involve a firearm" during the seven-year review period. Based on that, the report concluded that an estimated 10%-21% of people denied gun purchases -- whose cases made their way to ATF field offices -- committed "subsequent crimes (that) involved firearms" during the span of the study.

If the decade-old study's findings hold up, it suggests that anywhere from 1,322 to 2,631 of the 12,710 people who attempted to buy a gun in fiscal year 2017, failed a background check, and had their cases sent to ATF field offices, would eventually get a gun and commit a crime. Two criminal policy academics who reviewed the ATF paper at CNN's request said they were confident in the research paper's methodology and conclusions, and they think the rate of subsequent gun crimes by attempted purchasers would likely be the same or higher today.

Chipman thinks the problem may have gotten worse. Since 2006, online exchanges that connect buyers and sellers have changed the marketplace, making it easier than ever for a prohibited person to find a private seller and avoid a background check when buying a firearm. Federal law requires licensed gun shops to conduct background checks, but not private sellers.

And states have a difficult time cracking down on a burgeoning phenomenon: mail-order gun parts and do-it-yourself "ghost gun" kits, like the ones that allowed Zawahri to construct his own firearm in California. Webster, the Johns Hopkins professor, noted that private gun ownership in general has significantly increased, flooding the market with weapons that criminals may obtain through theft or undocumented sales.

Since the 2006 study, ATF has not assessed the threat from people who lie on gun background check forms, the agency told CNN. And the original paper remained unknown to the public, as is typical for such internal briefing papers. Asked why it was not released, ATF spokeswoman April Langwell told CNN: "The draft report to which you refer is now 12 years old. I do not know why it was not finalized/released ... & obviously the individuals who were working then are long gone."

Chipman, who commissioned the briefing paper, said that the report was final and not a draft. "The biggest frustration I ever had working at ATF is that some of this high-level data about what happens after people are denied" never reaches the public, he said.

"It really is something the people need to know," he said. "Facts matter. There's science and data. And that data should be out there ... it solidifies what the real challenges are so that policymakers have a shared group of facts."

New interest in lie-and-try; but so far, little change

The GAO investigator who recently reviewed prosecution of "lie and try" cases told CNN she was surprised to learn about the internal ATF statistics on subsequent crimes committed by people who provide false information on gun buying forms. The GAO was not aware of the briefing paper, GAO's Goodwin told CNN. Even without that information, however, the GAO acknowledged that a major crackdown would require more resources for law enforcement.

The GAO found that ATF agents sometimes simply send denied purchasers a text message or letter to deter future attempts -- and potentially serve as future evidence that the person knew they were prohibited. ATF officials told GAO they prefer text messages over official-looking warning letters because text messages "are less intimidating to prohibited persons, the texts save time and money, and are more effective in helping retrieve firearms."

The GAO report suggests that ATF review the benefits of using text messages and warning letters "as an enforcement tool," and perhaps consider making this an agency-wide policy. An ATF spokesman said there's no rigid policy at the agency for making contact with those who lie and try. Less serious cases, like those involving someone with a decades-old felony who tried to buy what's considered a turkey-hunting shotgun, result in a simple courtesy phone call to inform a rejected gun buyer why they were denied. More serious cases, like those involving domestic abusers who try to buy a handgun shortly after conviction, might result in a formal warning letter by mail or even a knock on the door.

In a country that has become increasingly polarized into Second Amendment rights and gun control camps, lie-and-try enforcement could be one area of consensus. The Democratic congressman who commissioned the report, US Rep. José E. Serrano of the Bronx in New York, told CNN of its release: "This is one rare moment where we can agree on guns."

"The laws we have should be enforced. We don't fund these agencies properly," he said. "The end result is that people are getting guns who shouldn't get guns."

A bipartisan group of 10 senators led by US Sen. Pat Toomey, a Republican from Pennsylvania, introduced a bill in March that would force the FBI to notify local law enforcement whenever a prohibited person tries to buy a gun and fails the background check. The NICS Denial Notification Act would not provide additional funding for local police to investigate gun form lying, but it would put lawbreakers on their radar.

In a statement to CNN, Toomey called his bill "a commonsense, yet critical, step Congress should take to strengthen gun safety laws."

"One thing that both parties agree on when it comes to gun safety is that we should enforce the laws that we already have on the books," US Sen. Chris Coons, D-Delaware, a co-sponsor of the bill, told CNN in a statement.

The bill has been referred to the Senate Judiciary Committee. The House version of the bill is in a similar stage. A previous version of the bill, as written by US Rep. Mike Quigley, a Democrat from Illinois, failed to make it through the House in 2017.

The lobbying power that most fiercely opposes new gun laws, the National Rifle Association, told CNN it does not oppose the bill. As long as 18 years ago, the organization was calling for tougher crackdowns on prohibited possessors who attempt to buy firearms, framing the issue this way: " prosecution is prevention ."

"The National Rifle Association continues to support the prosecution of dangerous people who knowingly attempt to illegally purchase a firearm," said Lars Dalseide, an NRA spokesman.

Because the bill lacks new funding to pay for local officers to actually follow up on the additional criminal data, former ATF agents tell CNN they doubt the bill will be effective. A spokesman for Coons said the senator would be willing to support a funding provision for the bill "if more resources are required to do the job correctly."

In March, former US Attorney General Jeff Sessions issued a memo to step up enforcement of lying on gun forms, directing all of the country's US attorneys to "enhance prosecution" in each of their districts "to ensure vigorous and appropriate prosecution of 'lie-and-try' cases." The memo still puts "particular emphasis on cases against violent persons," such as those with previous convictions for violent felonies and misdemeanor crimes of domestic violence.

"We are proud to have taken the most aggressive action to step up lie-and-try prosecutions in nearly a decade," a DOJ spokesman told CNN. He added that, under Sessions, the department has stepped up gun prosecutions in general, evident because it has "charged the highest number of federal firearm defendants in history this past year." In fiscal year 2018, the federal government charged more than 15,300 people with firearms offenses -- 17% higher than ever before, according to the DOJ.

The attorney general's directive came as the GAO was ending its review, so the report didn't capture a possible shift on prosecutorial standards. Sessions ordered prosecutors in the nation's 94 judicial districts to meet with ATF and revise local prosecution guidelines "as necessary." These guidelines are important, because they determine whether the feds in each district are willing to pursue charges -- and whether the bar could be higher, say, in Miami than in San Francisco.

When asked by CNN, however, ATF could not point to any change in the way its agents approach these types of cases since the directive from Sessions. The agency indicated that it had "met with all (US attorney's offices and) reviewed the guidelines."

Even to find out what those prosecutorial standards are, and how they differ by region, is no easy task. In January, CNN filed a public records request to ATF for copies of the guidance it receives from local prosecutors about which cases to refer. ATF has not yet formally responded, but a spokesman redirected CNN to the Executive Office for the US Attorneys. CNN filed a similar request with that agency in June. It redirected CNN back to ATF.

Some states have begun to recognize what a danger sign it can be for a prohibited person to skirt the law and try to buy a gun. Last year, Washington became the first state that notifies victims of domestic violence when their convicted abuser attempts to purchase a firearm. Gun dealers there must notify a state association of police and sheriffs whenever someone fails a background check, and the state's "automated protected person notification system" then passes on the information. Domestic violence groups in other states are taking notice.

"Too often our system turns a blind eye to how dangerous a case could be," said Tara Muir, policy director at the Wyoming Coalition Against Domestic Violence and Sexual Assault. A prohibited person's attempt to buy a gun "has to be one of the most dangerous indicators there is."