More than a decade ago, Eric Harris and Dylan Klebold opened fire on their classmates at Columbine High School using a semiautomatic rifle, pistol and shotguns they purchased through acquaintances who had visited a local gun show. There was no required record of the sale and no background check; just an exchange of cash and a handshake.
In March 2010, John Patrick Bedell strolled up to the Pentagon and started shooting at two police officers with a semiautomatic handgun. Months before the attack, he tried to buy a gun in California but was denied, after a background check showed he had a documented history of mental illness. So Bedell instead went to neighboring Nevada, where gun laws are more lenient, and bought a 9mm handgun from a private seller who didn't have to check out his history.
Both incidents point to a major loophole in the nation's system of regulating firearms, experts say: Private parties can buy and sell guns in many parts of the country with little or no scrutiny from state and federal authorities. Nearly 40 percent of gun transactions in America occur through so-called private party sales, creating a secondary firearms market that is largely invisible.
More than three-quarters of states have no laws requiring background checks or documentation during private party sales, increasing the risk of weapons falling into the hands of convicted felons, juveniles or those who are mentally ill. As lawmakers in Washington examine gun control measures in the wake of last week's school massacre in Connecticut, many advocates and researchers are pushing to extend federal regulations requiring background checks and registrations to private gun sales.
"Fixing this would be one of the single most important things we could do to address overall gun violence," said David Kennedy, director of the Center for Crime Prevention and Control at John Jay College of Criminal Justice in New York. "A lot of people don't understand that this is the way the world works. It means that people who everybody agrees shouldn't get guns have little trouble getting guns."
Under the current system, federal law on gun purchases extends only to the first point of sale. Federally licensed firearms dealers are required to perform background checks on prospective buyers to screen out those with felony records, a history of domestic violence or mental illness and several other categories. Dealers are also required to keep detailed records of customers.
On private party sales, none of those restrictions apply under federal rules. States come up with their own laws governing the secondary gun market, and the restrictions vary widely, leaving an uneven patchwork of regulations from state to state.
A handful of states, including California and Rhode Island, require universal background checks for all private party transactions. If someone wanted to purchase from an unlicensed seller at a gun show or anywhere else, he or she would have to go to a federally licensed seller to certify the transaction: the licensed dealer would have to perform a background check before the sale could go through and keep a record of the transfer.
Other states, including Illinois, require that sellers register the transaction with the state, though they don't require background checks for all private sales (Illinois requires background checks at gun shows). States such as Hawaii require all gun purchasers, including in the private market, to do a background check and obtain a gun license, which lasts for 10 days.
The vast majority of states have no regulations whatsoever on private gun sales, except for vague statutes that prohibit "knowingly" selling firearms to someone who is a criminal or has mental health problems.
Experts argue that the laws create incentives for buyers and sellers in private transactions to find out as little as possible about one another.
"Everybody understands how the game is played: the seller never asks any questions," said Dr. Garen Wintemute, who has researched the gun market as director of the Violence Prevention Research Program at the University of California-Davis Health System. "At a gun show the sign on the table says 'private sale,' and everybody knows exactly what that means. It means no waiting period, no background check, no questions asked, cash and a handshake, and you're gone."
In a state such as California, where background checks and registration transfers are required on all private transactions, advocates say the law holds the seller accountable. If a gun was recovered at a crime scene and traced back to the seller, instead of the criminal, the seller could be held liable for an illegal gun sale.
But in states where there are lenient laws for private gun sales, critics argue that authorities have very little leverage.
"We've essentially legalized that type of activity," said Ladd Everitt, a spokesman for the Coalition to Stop Gun Violence. "The only way you would be able to convict the seller is if you can prove that he knew the buyer was a convicted felon."
Other experts argue that regulating private gun sales wouldn't address questions about the vast number of stolen guns, or black-market firearms networks that are already established.
"You would eliminate the category of private sales where an honest person unwittingly sold a gun to someone who seemed trustworthy but was not," said Nicholas Johnson, a law professor at Fordham University who has researched firearms laws. "But the people you're worried about are the people who are trading guns already in the black market. I don't think anyone should have any illusions about this solving the problem."
The exceptions for private gun sales in federal law date back to 1968, when Congress required that those "engaged in the business" of selling guns get federal licenses. But the law did not apply to those who occasionally sold firearms at gun shows or other informal gatherings.
Congress passed the Brady Handgun Violence Prevention Act in 1993, which mandated background checks on all firearms purchasers. But once again, the requirements only applied to federally licensed dealers.
After the Columbine High School shootings in 1999, President Bill Clinton and many in Congress pushed to close what they called the "gun show loophole." The two shooters at Columbine, Harris and Klebold, used weapons that had been obtained by friends at a gun show where there were no background checks required.
One of the friends, Robyn Anderson, testified that she would not have purchased the guns if she had to submit to a background check.
In 1999 the Senate narrowly passed a bill that would have required mandated background checks for all purchases at gun shows. But the Republican-controlled House, under pressure from the National Rifle Association, balked at the measure.
The NRA spokesman at the time, Bill Powers, also argued that background checks wouldn't have deterred the killings at Columbine.
"There's something far more troubling going on than whether we have an instant check at a gun show," Powers told The Denver Post.
In an interview with Rolling Stone at the end of his presidency, Clinton recalled being astounded that legislation requiring background checks at gun shows never succeeded.
"I thought Congress would be so shocked and the public was so galvanized that we had a window of opportunity," Clinton said in the 2000 interview. "The Republican leadership just delayed until the fever went down. That's what happened."
White House spokesman Jay Carney said this week that Obama would support legislation closing the so-called gun show loophole on background checks. Many members of Congress have come out in favor of reforms this week, with several changing their stance on gun control. Sen. Dianne Feinstein (D-Calif.) said she would be introducing legislation to reinstate the assault weapons ban at the beginning of next year, a move that the president supports.
Congress has introduced more than a half-dozen bills to require background checks for gun sales over the past 10 years, but the bill only made it out of committee once. In federal lobbying reports over the past decade, the NRA has listed gun show bills as a central lobbying target.
The NRA did not respond to requests for comment, but made its first public statement Friday in response to the Connecticut shootings. Instead of addressing calls for reform, the group blamed video games and the media for inciting violence, and suggested that schools should have armed guards to prevent future attacks.
The group Mayors Against Illegal Guns, co-chaired by New York Mayor Michael Bloomberg, an influential gun-control voice, has long made mandatory background checks for private gun sales a top priority for reform.
This week, the group sent a letter to President Obama outlining gun control proposals. First on the list was to address "the enormous gap in our laws" by requiring mandatory background checks on private gun sales.
Although the NRA has historically opposed such measures, public opinion may be shifting. A poll of NRA members and gun owners, conducted earlier this year by GOP pollster Frank Luntz, found that 74 percent of NRA members and 87 percent of non-NRA gun owners supported mandatory background checks for all gun purchases.
Wintemute, the UC-Davis violence expert, said the data shows there can be common ground on the fractious topic of gun control, if the debate shifts from rhetorical extremes to concrete proposals for change.
"The very term 'gun control' sets itself up for trouble,'" he said. "We use the term 'control' for things like 'crime control' and 'pest control.' But guns aren't pests. They have legitimate use. We're simply trying to regulate that use in ways that the vast majority of gun owners support."
This story has been updated to add context from developments on gun control reform this week.