Stephen Paddock's lethal attack on a Las Vegas country music festival Sunday night was distinguished from most mass shootings by two features: the size of the arsenal he smuggled into the Mandalay Bay Resort and Casino and the great height from which he shot.
With a cache of 17 mostly powerful firearms, Paddock, 64, smashed the windows of his 32nd floor hotel room and then, from high above the Las Vegas Strip, sprayed bullets down on 20,000 people listening to country music star Jason Aldean.
One of the weapons Paddock apparently used in the deadliest mass shooting in modern U.S. history was an AK-47 type rifle, with a stand to steady it for firing, according to people familiar with the case.
Investigators believe at least one of the guns functioned as if it were fully automatic and are now working to determine if he modified it or others to be capable of spitting out a high volume of fire just by holding down the trigger, people familiar with the case said.
"The ATF hasn't evaluated them yet," Las Vegas sheriff Joseph Lombardo said at a news conference Monday evening.
But video from the attack suggests Paddock may have used at least one fully automatic rifle, marking the first time such a weapon has been wielded by a public mass shooter in the United States, experts said.
"I really can't recall another case where one has been used," said James Alan Fox, a Northeastern University criminologist who studies mass shooters and believes Paddock was armed with an automatic weapon when he killed at least 59 people and set off chaos that injured more than 500.
"It doesn't take that much imagination to know what automatic gunfire sounds like," Fox added. "Anyone who has seen a war movie or an 'Untouchables' episode knows what it sounds like."
To Fox and others, the Las Vegas rampage represents a frightening turn in the nation's struggle to stop mass shootings.
In addition to Paddock's choice of weaponry, mass shooting experts were struck by his decision to shoot at the concertgoers from high above, rather than walking through the crowd or firing from a nearby secluded spot on the ground.
The tactic echoed two earlier mass shootings, most notably Charles Whitman's 1966 attack from atop the University of Texas Tower. The ex-Marine sharpshooter killed 17 and wounded more than 30, including a pregnant student shot in the belly.
A decade later, Michael Soles, a 19-year-old dumped by his girlfriend, chose the top of a Holiday Inn in Wichita, Kan., to shoot and kill three people while wounding eight others.
But Paddock was far higher than either Whitman or Soles. Lombardo estimated that he was shooting at victims at least 500 yards away.
Investigators are working to determine if Paddock modified the AK-47 with mechanical components to make it fully automatic, which is illegal, or if he used a legal modification like an attachable crank, which depresses the trigger faster than a finger and can be bought online for as little as $40.
Automatic firearms have been heavily restricted in the United States since 1986, when Congress passed the Firearm Owners' Protection Act.
Under the law, such weapons made prior to 1986 can be legally owned following a stringent background check and registration of the gun. Last year, the ATF sent a letter to the National Firearms Act Trade and Collectors Association saying that 490,664 automatic weapons were registered in a government database.
Gun owners with technical know-how can illegally modify rifles to make them automatic. But the industry also sells legal add-ons that can make semiautomatic rifles such as the AR-15 mimic automatic fire.
Gun purchase records indicate Paddock legally bought more than two dozen firearms over a period of years, according to a person close to the investigation.
Guns & Guitars, a shop in Mesquite, Nev., released a statement acknowledging that Paddock bought guns at the store, but passed "all necessary background checks. He never gave any indication or reason to believe he was unstable or unfit at any time. We are currently cooperating with the ongoing investigation by local and federal law enforcement in any way we can."
Lombardo said Paddock brought the weapons to his hotel room, probably concealed in a duffle bag or some other way.
When he began firing, Paddock's victims were confused, defenseless and easy targets as police scrambled to find where the shots were coming from. Las Vegas first responders found Paddock after smoke from his rifles set off fire alarms in his hotel room.
Adam Lankford, a University of Alabama criminologist who studies mass killers, said attacking from such a high location gave Paddock a tactical advantage that "rendered moot" everything Americans have been taught to do in mass shooting situations - to run from the gunfire, hide or even fight back if encountering the shooter.
"The people below are at a clear disadvantage," Lankford said. "They're helpless."
Paddock also had the advantage of time.
The vast majority of active shooter incidents end within five minutes, with the shooter being stopped by police, bystanders or their own suicides, according to an FBI study of attacks between 2000 and 2013.
Paddock is believed to have killed himself as police closed in, but had ample time before officers arrived to reload and switch rifles if one overheated or malfunctioned. Lombardo said he had used multiple weapons.
Fox, the Northeastern University professor, said the apparent automatic-style firing combined with an elevated shooting position is what led to the extraordinary victim count. If Paddock had been on the ground, Fox said, "he still would have killed a lot of people, but he would have been overtaken more quickly."
Now, for experts who study these horrifying events, there is concern about what happens next.
There have been studies suggesting that contagion plays a crucial role in mass shootings — that one shooting leads to another. Will other shooters go for automatic firepower? Will tall buildings become coveted shooting perches?
Fox is concerned about those issues, but he's most troubled by news organizations repeating that Paddock's attack is the worst mass shooting in modern U.S. history. Mass shooters typically revere their predecessors and often leave writings or other evidence (charts, lists, etc.) that show attempts to set new killing records.
"Why do we keep such records?" Fox said. "That's more of an enticement to follow in his footsteps than anything else."
The Washington Post's Tom Jackman and Sari Horowitz in Washington and Heather Long in Las Vegas contributed to this report.