In the wake of the recent terror attacks in France, the fact that someone on a terrorist watch list can still pass a background check and buy a firearm from a licensed gun dealer is making media headlines. But terrorists and other dangerous individuals don't actually need to subject themselves to the scrutiny of a background check because of a loophole in federal law.

Media discussions on how a terrorist might get a gun in the U.S. have largely centered on what is known as the "terror gap." Under current federal law, individuals who are on terror watch lists are not prohibited from buying firearms. According to a report from the Government Accountability Office, more than 2,000 people on the FBI's consolidated terror watch list were approved to purchase firearms between 2004 and 2014, despite the fact that they underwent background checks. The National Rifle Association opposes barring individuals on this list from buying firearms, arguing that doing so would violate Second Amendment rights.

But potential terrorists don't need to submit themselves to a background check at all. Due to a loophole in federal law, a significant number of gun sales can occur without a background check, even to those on the terror watch lists.

The federal background check law only requires individuals “engaged in the business” of selling firearms to obtain a license and perform background checks on customers. People who are engaged in “occasional sales” or sell out of their “personal collection” do not need to obtain a Federal Firearms License (FFL) or run checks on buyers. (Eight states have closed this loophole by enacting laws requiring a background check at the point of sale for all firearms.)

Some so-called “private” gun sellers, including firearms traffickers, take advantage of the vagueness of the definition of what it means to be “engaged in the business” of selling firearms in order to sell large numbers of guns without a background check. These types of sales occur at gun shows, and increasingly over the Internet.

Terrorists have already been caught exploiting gun shows to obtain weapons. In 2001, the New York Times reported on the conviction of a Hezbollah member who attempted to divert weapons from gun shows in Michigan into Lebanon. In June 2015, a North Carolina teenager who was arrested by the FBI was allegedly planning on obtaining an assault weapon from a local gun show to use in an ISIS-inspired attack.

A 2011 undercover investigation by the City of New York of seven gun shows in three states found that 19 out of 30 private sellers agreed to a sale where the buyer said he probably couldn't pass a background check. One seller who was surreptitiously filmed sold a gun to an undercover investigator who told him three times that he couldn't pass a background check. Other sellers simply laughed and continued with the sale when the investigator said he couldn't pass a check:

An investigation of online sales in 2011, also by the City of New York, found a similar trend, with 62 percent of sellers agreeing to complete a sale to someone who said he or she probably couldn't pass a background check.

Al-Qaeda is aware of the private sale loophole, and has urged its followers to exploit it. In a 2011 video, American born al-Qaeda propagandist Adam Gadahn urged al-Qaeda's followers to go to gun shows in order to buy firearms without undergoing a background check, asking his audience, “So what are you waiting for?”