Military-style assault weapons like the one Omar Mateen used to blast away at an Orlando nightclub remain legal and easy to purchase — easier, in Florida, than buying a handgun.
Mateen wielded an AR-15, the most popular rifle sold in the United States according to the National Rifle Association. Manufactured by companies such as Colt and Bushmaster, it's the civilian version of the M-16 rifle used by the U.S. military and many armies around the globe.
President Barack Obama and other gun control advocates have repeatedly called for reinstating a federal ban on semi-automatic assault weapons that expired in 2004. But they have been thwarted in Congress by majority Republicans and some pro-gun Democrats.
Opponents of a ban say semi-automatic rifles and pistols are good for hunting, target shooting and self-defense, and that a prohibition would infringe on the constitutional right to bear arms while doing little to prevent crime. Both sides agree that such weapons are used in only a tiny fraction of gun crimes.
But gun control advocates say any legitimate use of the weapons pales next to their ability to quickly inflict mass casualties.
"If you have a fairly clean record and a credit card, you can build your own army," said Josh Sugarmann, executive director of the Violence Policy Center.
"Sometimes what gets lost in the semantics of this debate is the fact that these are firearms that were designed to kill as many people as possible" as quickly as possible, he said.
Representatives of the National Rifle Association and the National Shooting Sports Foundation, the firearms industry's trade group, did not return messages seeking comment.
In the wake of the Sunday morning shootings at Orlando's Pulse night spot, the presumptive presidential candidates spoke out on different sides of the issue.
Democrat Hillary Clinton has called for reinstating the assault weapons ban, saying "military-style assault weapons do not belong in our streets" and pose a threat to police officers. "We have to make it harder for people who should not have those weapons of war," she told a Cleveland audience on Monday.
Republican Donald Trump opposes an assault weapons ban, though in 2000 he said he supported it. "People have to be able to protect themselves. And the bad guys have them, so if the bad guys have them, we need protection in this country," he told CBS "This Morning" on Monday.
Fully automatic weapons are illegal for civilian use; they can fire many bullets with a single pull of the trigger. The triggers on legal, semi-automatic weapons fire a single bullet each time.
Still, the weapons can be fired rapidly — as fast as one shot per second — because they feature grips for shooters to control them and detachable magazines that can carry 30 rounds or more. Their retail prices vary but are often around $1,000.
Florida law sets a three-day waiting period for purchase of handguns, often used in crimes. But the state mandates no waiting period for any gun that requires two hands to hold.
Spurred partly by the killings of five schoolchildren and the wounding of 29 others and a teacher in Stockton, California, in 1989, Congress in 1994 banned certain semi-automatic assault weapons and high-capacity ammunition magazines that held more than 10 rounds. But the law expired in 2004, and hasn't been renewed despite polls showing popular support for the idea.
Researchers have disagreed on whether the ban had a modest impact on gun crime, or none at all. Manufacturers were able to continue making similar weapons with slight design changes.
But the debate gained renewed energy after the December 2012 massacre at Sandy Hook Elementary School in Newtown, Connecticut, when Adam Lanza used his mother's semi-automatic assault rifle to kill 20 first-graders and six adults. An attempt to impose a federal ban on assault weapons failed the next year, when Republicans in the U.S. Senate joined with some Democrats to block the proposal.
Since then, similar firearms have been used in other mass shootings, including the attacks in San Bernardino, California, at a military center in Chattanooga, and an Oregon community college.
Mateen, 29, was armed with an AR-15 and a handgun when attacked the crowded nightclub in the worst mass shooting in U.S. history, which claimed at least 49 lives. Dozens more were injured before Mateen was killed by police.
Mateen, a security guard who had been investigated by the FBI for possible ties to terrorists, had legally purchased weapons in recent days.
In December, House Democrats introduced a new federal ban on assault weapons that was designed to address the shortcomings in the earlier law. The plan failed to advance in the Republican-controlled chamber.
Some states have tried to impose their own restrictions on the weapons or sought measures that would at least slow down shooters using them.
Last month, California's Democratic-controlled state Senate voted to outlaw the sale of assault weapons with easily detachable magazines and to require that people turn in ammunition magazines capable of holding more than 10 rounds.
Eight states have banned certain assault weapons, the use of high-capacity ammunition magazines, or both, according to the Law Center to Prevent Gun Violence. Requiring a shooter to reload after every 10 rounds fired would prevent at least some carnage in mass shootings, supporters of such measures say.