A week after Omar Mateen gunned down 49 people and injured more than 50 others at a gay nightclub in Orlando in June, U.S. House Speaker Paul Ryan, R-Wisconsin, appeared on NBC’s "Meet the Press."

Days earlier, the FBI disclosed that Mateen had previously been under investigation and was, for a time, on a terrorist watch list.

Ryan was asked about legislation intended to delay or prevent people on those lists from obtaining guns.

"The question right now," Ryan said, "is if someone is on a terror watch list, are the authorities notified as to whether a person on that list is trying to purchase a gun or not? That is the procedure right now."

Is Ryan right that under the current procedure, if someone "on a terror watch list" tries to buy a gun, authorities are notified?

How gun screening works

When we asked Ryan’s office to provide backup for his claim, a spokesman pointed to a 2010 U.S. Government Accountability Office report. That report states the FBI is alerted when a federal background check for a gun transfer is performed on someone whose name is on a terrorist watch list.

We confirmed the same with Dave Joly, a spokesman for the FBI’s Terrorist Screening Center, and Second Amendment expert Adam Winkler, a law professor at the UCLA School of Law.

Such background checks are mandated under a federal law called the Brady Handgun Violence Prevention Act — commonly known as the "Brady Bill."

States can choose to conduct the background checks themselves, or have the FBI perform them. Either way, a background check involves verifying the buyer’s identity and searching for information in databases that would determine if the gun purchase is illegal.

For instance, someone who is under indictment, has a felony conviction, uses illicit drugs or is an illegal immigrant is not permitted to buy a gun. Background checks typically result in approving the gun purchase, denying it or imposing a three-day delay to allow for further investigation.

While conducting such checks, the federal government can screen for people whose names appear on terrorist watch lists — though the fact someone is on such a list does not alone disqualify that person from buying a gun under the Brady Bill.

Indeed, the vast majority of people on terror watch lists have been allowed to purchase guns.

The federal government began cross-referencing gun background checks with terrorist watch lists in 2004, and through 2015, there were 2,477 instances when someone on a terrorist watch list was involved in a gun background check. About 91 percent of those transactions were permitted, according to the Government Accountability Office.

If federal authorities are notified someone on a terrorist watch list is trying to buy a gun, they can take steps to thwart the purchase. For instance, investigators can review paperwork the prospective gun buyer completed to check for other new, potentially disqualifying information.

Private sales excluded

Importantly, the Brady Bill does not require background checks for all gun transfers: It applies only to gun sales by federally licensed firearm dealers. Unlicensed sellers are not required to document or report sales under the law.

At least 18 states and the District of Columbia have enacted laws that go beyond the Brady Bill and require background checks in some private gun sales, according to the Law Center to Prevent Gun Violence, a nonprofit organization that tracks firearm laws and litigation in the United States and seeks to reduce gun violence.

But the other states — including Wisconsin — have no such requirement.

There is no definitive figure of how many private gun sales occur, although experts have made some informed estimates.

A National Institute of Justice study estimated there were 13.7 million gun transfers between 1993 and 1994 in the United States; roughly 60 percent involved licensed dealers, while about 40 percent were "off-the-books transfers" through unlicensed sellers.

Preliminary data from a more recent, yet-to-be-published survey suggests the proportion of unlicensed gun sales has decreased, but such sales still involve millions of guns each year, according to Northeastern University Professor Matthew Miller, who co-directs the Harvard Injury Control Research Center.

Our rating

Ryan said that under the current procedure, if someone "on a terror watch list" tries to buy a gun, authorities are notified.

That is accurate as it applies to gun sales in which a federal background check occurs. But a significant amount of gun sales or other transfers lack background checks, so there would be no red flag when someone on the list tries to buy a gun.

For a statement that is partially accurate, but leaves out important details, we rate Ryan’s claim Half True.