WASHINGTON — Omar Mateen, investigated twice by the FBI, was on the government's terrorist watch list for 10 months before being removed. Yet even had he remained on that listing, it wouldn't have stopped him from buying the firearms he used in Sunday's Orlando shooting rampage.
Senate Democrats are hoping to use that little-known fact and the worst mass shooting in modern U.S. history to pressure Republicans to take what could be a politically painful election-year vote to curb gun sales.
The FBI investigations, in 2013 and 2014, closed with no charges against Mateen, 29. Yet the day after the attack by the American-born Muslim left 49 people dead and more than 50 others wounded in a gay nightclub, President Barack Obama and FBI director James Comey said he was probably inspired by foreign terrorist groups. Mateen died in a gunfight with a SWAT team.
A look at the intersection between the terrorist list, guns and the Orlando bloodbath:
Q: What is the terrorist watch list?
A: The FBI's Terrorist Screening Center, created in 2003 following the 9/11 attacks on the U.S., maintains the terrorist watch list, a database of people known or suspected of being involved in terrorist activities. Federal agencies used to keep nearly a dozen separate listings but shared them only occasionally.
The list had around 800,000 names on it in 2014, according to testimony the Center's director, Christopher Piehota, gave that September to a House subcommittee. The no-fly list, a subset of the broader terrorist listing, has around 64,000 people on it, Piehota said.
Q: That's a lot of people.
A: It is. But the FBI notes that only about 2 percent of them are U.S. citizens or legal permanent residents allowed to buy guns. The rest are foreigners, many of whom are not permitted to purchase firearms here.
Q: Why can people on the terrorist watch list buy guns?
A: That's the law. Being on a terrorist watch list is not "in and of itself a disqualifying factor" for people purchasing firearms and explosives, according to a 2013 report by the nonpartisan Congressional Research Service.
People purchasing guns from federally licensed firearms dealers must undergo background checks, and they can be denied if they fall into any of 10 categories. These include convicted felons or drug abusers, people found by courts to have certain mental problems and immigrants in the U.S. illegally.
The FBI is notified when someone on a list applies to purchase a gun, often resulting in increased surveillance of a suspect. But being a suspected or known terrorist is not one such category.
People don't need background checks to buy guns from unlicensed sellers, such as from some who offer firearms at gun shows or online. It's unclear exactly how many guns are sold that way.
The FBI also conducts background checks on people applying for licenses to ship or receive explosives.
Q: Are there many sales to people on the terrorist watch list?
A: Sen. Dianne Feinstein, D-Calif., chief sponsor of legislation that Democrats are pushing following Orlando, got numbers in March from the nonpartisan Government Accountability Office, Congress' investigative arm.
GAO said from February 2004, when the background check system began monitoring the terror watch list, through 2015, 2,477 names of would-be gun and explosives buyers were on the watch list. Nearly all were for gun purchases.
Of those, 91 percent, or 2,265, of the transactions were permitted.
For comparison, the FBI and states conducted more than 23 million background checks last year for gun purchases, the most ever.
Q: What would Feinstein's bill do?
A: It would let the attorney general deny firearms and explosives to people known or suspected of being involved in terrorist activities. It's not necessarily based on the government's terrorist list.
The Senate rejected it last December by a near party-line vote, and barring an unexpected compromise the same fate likely awaits it.
"Life and death, that's the reason," Feinstein said Tuesday about why the outcome might be different with a fresh vote. "How much do we want to go through?"
Last year's vote occurred a day after an extremist couple killed 14 people in San Bernardino, Calif. Nearly all Republicans opposed Feinstein's proposal, saying owning guns is a constitutional right and noting that some people have been erroneously suspected as terrorists.
That same day, senators derailed a proposal by Sen. John Cornyn, R-Texas, that was opposed by most Democrats and would have let the government delay firearms sales to suspected terrorists for up to 72 hours. The transaction could be halted permanently if officials could persuade a judge to do so.
Democrats said clever lawyers could easily delay court action for 72 hours, rendering Cornyn's proposal toothless.
Cornyn told reporters that Democrats seem "more interested in opportunistically using this tragedy to advance their agenda" than addressing guns.
Along with many Republicans, Senate Majority Leader Mitch McConnell, R-Ky., has focused his response to Orlando on the need to beef up defense, intelligence and law enforcement efforts against extremist groups like the Islamic State.
Associated Press writer Alicia A. Caldwell in Washington contributed to this report.