The murder of 58 people at a country music festival in Las Vegas has reopened America's debate about gun laws (even if there's little prospect of anything changing).
Stephen Paddock fired on the crowd of 22,000 people from more than 300 metres away, from the window of his 32nd-floor hotel room.
Here's what we know about the weapons he used to do this and how he may have been able to access them.
Police say they found 23 guns in his hotel room
That's on top of the 19 firearms and thousands of rounds of ammunition found at Paddock's home in nearby Mesquite, Nevada.
A search of his car also turned up a supply of ammonium nitrate, the fertilizer used to form the explosives used in the 1995 Oklahoma City bombing of a federal office building that killed 168 people.
The firearms found in Paddock's hotel room had calibres ranging from .308 (used in hunting rifles, for example) to .223 (used in AR-15 semi-automatic rifles), and a handgun was also among the weapons.
Police believe Paddock smuggled the weapons into the hotel using more than 10 suitcases, which he methodically brought to the hotel room one-by-one.
Paddock may have had fully automatic weapons — or guns legally modified to act like them
A law enforcement official, speaking to Reuters on condition of anonymity, said authorities believe Paddock had at least two such weapons.
Clark County Sheriff Joseph Lombardo confirmed that officials believe at least some of the guns found in the hotel room were modified, possibly to allow for more rapid fire.
Fully automatic firearms can fire as many rounds as their magazines, drums or belts hold by simply pulling and holding the trigger — whereas semi-automatic weapons require a trigger pull for each round fired.
Police at the scene of the shooting had reported the gunman was using an automatic weapon because of the "rat-a-tat" sound of the gunfire.
Automatic weapons were banned in America more than three decades ago, but bump stocks get around this
Two officials told AP that authorities found two bump stocks in Paddock's hotel room and were investigating whether they were used to modify weapons used in the massacre.
Originally created with the aim of making it easier for people with disabilities to shoot a gun, bump stocks allow semi-automatic rifles to mimic fully automatic weapons by unleashing an entire large magazine in seconds.
A weapons expert told the Washington Post that images from the hotel room showed an AR-15 type rifle that had been modified with a bump stock:
An official also told the New York Times that AR-15-style assault rifles had been found among the weapons in the hotel room.
An AK-47 type rifle was also reportedly found.
Bump stocks allow for faster rates of gunfire
Bump stocks work by replacing a semi-automatic rifle's stock, which rests against the shoulder to provide stability and absorb recoil.
They cause the gun's recoil to instead press against the shooter's finger after each shot, which means rounds can be fired much more quickly than is possible by pulling the trigger manually.
Bump stocks have been around for less than a decade, and the US Government gave its seal of approval to selling them in 2010.
They were deemed legal because the trigger is still pulled for each round, even though the rate is faster than is possible using only one's finger.
Some Democrats have called for bump stocks to be banned.
As well, despite the ban on automatic weapons, there are still legal ways to buy them
Even though civilians are banned from buying or selling fully automatic weapons made after 1986, individuals can legally possess older weapons after passing a background check and obtaining a special permit.
There are about 176,000 pre-1986 machine guns registered with the US government that can be legally transferred, and they typically cost tens of thousands of dollars.
We don't know exactly when or how Paddock got his weapons
Chris Sullivan, owner of the Guns & Guitars shop near Paddock's home in Mesquite, Nevada, confirmed Paddock had bought firearms from the store.
However, he did not offer more detail, beyond saying Paddock had cleared "all necessary background checks and procedures".
"He never gave any indication or reason to believe he was unstable or unfit at any time," Mr Sullivan said.
Nevada allows residents to own machine guns if they are permitted under federal law. The state does not require licenses, registration or a waiting period for firearms, including semi-automatic rifles.