An American citizen who claimed allegiance to ISIS killed 49 people and wounded 53 at a gay nightclub in Orlando, Florida early Sunday. The gunman, 29-year-old Omar Mateen, was killed by police.

The attack drew renewed attention to the threat of home-grown terrorists and to gun laws that enabled Mateen to buy an AR-15-style rifle and a handgun, despite the FBI’s previous investigations into his potential terror ties.

“We have to counter extremism,” President Barack Obama said Monday. “But we also have to make sure that it is not easy for somebody who decides they want to harm people in this country to be able to obtain weapons to get them.”

New York Senator Chuck Schumer, who is set to lead the Senate Democrats next year, said that he hoped “the horrors of Orlando will bring Congress” to bar people on the terrorist watch list from buying a gun.

A study by Indiana State University researchers found that prior to the attacks of September 11, 2001, bombs were the most common weapon for terrorists carrying out attacks on American soil. But new regulations in the years that followed made it more difficult to obtain the chemical ingredients, which experts believe is one explanation for the rising popularity of high-powered guns as the weapon of choice.

As The Trace has reported, 95 percent of terrorism deaths in the United States since the 2001 attacks were from gunfire.

The ease with which suspected terrorists can legally purchase firearms is often referred to as the “terror gap.” The FBI maintains a terrorism watch list that includes an estimated 800,000 names, including U.S. citizens and foreigners, classified as those “known or reasonably suspected of being involved in terrorist activity.” But there is no federal prohibition preventing people whose names are on the list from buying guns.

Republican Senator John Cornyn is expected to re-introduce a GOP plan this week that would notify law enforcement officials if someone on the terror watch list tried to buy a gun, and give the Justice Department 72 hours to show probable cause that the would-be buyer has committed or will commit an act of terrorism. If the Justice Department tries to deny a purchase, the individual would also have the right to argue their case.

Who is currently prohibited from buying a gun?

Federal law prohibits nine categories of people, including convicted felons, from purchasing or owning firearms.

Mateen’s ex-wife claimed Sunday that he abused illegal drugs, suffered from mental illness, and engaged in domestic violence. Each of those issues, if they led to an involuntary commitment, restraining order, or otherwise met a legal threshold, could have legally barred him from buying a gun. But no federal law bars suspected terrorists from buying guns.

What’s the story behind the so-called terror gap?

After the September 11 attacks, federal law enforcement agencies created a database of suspected or potential terrorists. Some of those names would later appear on “no fly” lists.

Then-U.S. Attorney General John Ashcroft, a gun rights advocate, blocked the FBI from comparing federal gun-buying records against a list of suspects detained as part of the investigation into the 9/11 attacks. Ashcroft said federal law prevented using the background check information for other law enforcement purposes, The New York Times reported.

Two states, Connecticut and New Jersey, have instituted laws that block people on the watch list from buying firearms. Similar federal legislation, introduced by California Senator Dianne Feinstein after the ISIS-linked shooting in San Bernardino, California, last December, was defeated on a 45-54 vote. All Democrats except Heidi Heitkamp of North Dakota voted in favor of the bill. Mark Kirk of Illinois was the only Republican to cross party lines and support the legislation.

The Feinstein bill would have required the Justice Department to check would-be gun buyers against the government’s consolidated terror watch list. Estimates put the number of Americans on the list at between 10,000 and 40,000.

Feinstein said Monday she plans to “try again” to pass the measure. “We need the American people to support this,” she said on MSNBC. “The ease with which a potential terrorist can buy a weapon in this country is frightening.”

Federal agents receive an alert if a person on the watch list tries to buy a gun, but they cannot block the sale. On Monday, FBI Director James Comey wouldn’t elaborate on the fact that, when it comes comes to those like Mateen, who are no longer on the list, his agency does not get such alerts.

What are the objections to banning people on the terrorist watch list from buying guns?

The most cited concern is that a blanket ban would deprive people of a constitutional right without due process.

“This is not the way we’re supposed to do things in this country,” Cornyn said before Feinstein’s bill was voted down.

A ban would almost certainly apply to people whose names mistakenly appeared on the watchlist. The list is considered both bloated and imprecise. The National Counterterrorism Center reportedly rejected less than 1 percent of names submitted by law enforcement agencies in 2013.

Some pundits and groups on the left, like the American Civil Liberties Union, have also expressed due process concerns.

Would a ban on gun ownership by suspected or possible terrorists have prevented the Orlando shooting?

Mateen was temporarily put on the federal terrorist watch list maintained by the FBI in 2013 and 2014, according to the FBI. In 2013, agents questioned Mateen after his coworkers reported that he made statements sympathetic to terrorists.

The agents closed the case after determining Mateen broke no laws. The FBI questioned Mateen again in 2014 after learning he had contact with an American who died carrying out a suicide bombing in Syria. The agency again closed that second probe because Mateen’s contact with the suicide bomber appeared to have been minimal. In each case, Mateen was removed from the watch list after being cleared.

He was not on the watch list at the time he reportedly bought the guns he used to carry out the nightclub massacre. Which means that under the legislation Feinstein proposed, he would not have been barred from purchasing the weapons.

What will happen now?

Democrats say they plan to force a vote in the next few weeks on Feinstein’s bill in the Senate. Republican leaders said this week they expect their members will again block it.

Republicans argue the terror gap legislation is an electoral gambit aimed at creating an election-year wedge issue. “The real question is do they actually want to solve a problem or do they just want a talking point for a political ad,” Cornyn, the second-ranking Senate Republican, tells The Trace.

Democrats say the alternative bill Cornyn proposed has such stringent due process provisions that people on the list would remain able to buy guns unless federal officials could prove ties to terrorism in court. Democrats are likely to block the measure, leaving the law unchanged after the dueling votes.

While the “terror gap” is in place, how often are suspected terrorists legally buying guns?

The Government Accountability Office found that people on the terror list bought guns 2,043 times between February 2004 and December 2014. In 190 instances, about 10 percent of the total, a person on the list was barred from buying a gun due to some other disqualifying record.

[Photo: Lee Sternthal for The Trace]