From his hotel room on the 32nd floor of the Mandalay Bay Resort and Casino in Las Vegas, Stephen Paddock would have looked down upon a crowd of more than 20,000 people, surging to the final sets of a country music festival.
He opened fire late Sunday, killing at least 59 people and injuring 527 others in one of the deadliest mass shootings in American history, the authorities said.
But what may have seemed like a difficult feat, firing across an urban area and into a crowd from about 500 yards away â the equivalent of several football fields â appears to have been offset by Mr. Paddockâs preparations, which made it possible for him to inflict mass carnage.
Law enforcement officials cautioned that their information remained preliminary amid a rapidly unfolding investigation, and it was at times contradictory. But officials said Mr. Paddock established firing positions by smashing a pair of windows in his hotel room. He was armed with at least 17 firearms, the authorities said, including rifles designed to be fired at such distances. He was also perched from a vantage point that increased the likelihood that even errant shots were more likely to strike someone than had he fired them from ground level.
Among his weapons, a law enforcement official said, were AR-15-style rifles, a civilian variant of a standard service rifle used by the American military for more than a half-century.
The possibility that Mr. Paddock used tripods, which two law enforcement officials said were in the room, indicates that he understood how to overcome some of the difficulties of his plan. Special mounts designed to fit the underside of a rifle and sit atop camera tripods allow the gunman to fire more accurately while standing. Military snipers use tripods in urban spaces, often setting themselves back from a window so neither they nor their weapons can be seen from the streets below.
These preparations, along with the downward angle of Mr. Paddockâs gunfire and the density of concertgoers, would make the shooting more lethal than it might otherwise have been, and more difficult to counter or escape.
When the gunshots started, videos showed, those in front of the stage dropped to their stomachs â often an adequate first measure when under fire. But on Sunday night, the decision potentially put them at greater risk.
Mr. Paddockâs position overhead gave him a vantage point over objects and obstacles that would typically protect people from bullets flying from a gunman at ground level. It also meant that inaccurate shots â the sort common to rapid or hurried fire, which typically sail high or strike the ground short â could still plunge into areas where people were huddled.
Audio recordings of the shooting suggest that at least one of Mr. Paddockâs weapons fired automatically, discharging multiple bullets with a single depression of a trigger, in what are commonly called bursts.
Weapons capable of burst fire have long been federally regulated in the United States and are more difficult to obtain than weapons that fire semiautomatically, for which regulations vary by state.
It was not clear on Monday evening whether Mr. Paddock possessed such weapons, or used semiautomatic weapons that had been altered. In some videos of the shooting, the rate of fire sounds inconsistent, at times sputtering.
This suggests the possibility that a weapon could have been modified to fire more quickly, a change to semiautomatic firearms known as bump or slide fire. Such modifications harness the recoil to allow for rapid fire.
Sheriff Joseph Lombardo of Clark County, Nev., said that 16 rifles, ranging from .308 to .223 caliber, and a handgun were retrieved from Mr. Paddockâs hotel room. A federal law enforcement official said that AR-15-style rifles were among them. The authorities did not detail all of the guns, or which weapons Mr. Paddock fired.
Mr. Paddock had purchased some guns in Arizona, according to a gun seller there who spoke with the authorities.
Several pounds of a nonflammable exploding target used for practice were recovered from Mr. Paddockâs home in Mesquite, about an hour outside Las Vegas, Sheriff Lombardo said. Ammonium nitrate was found in Mr. Paddockâs car in Las Vegas, the sheriff said, but he did not say how much was recovered.
Determining which weapons were used will fall to investigators reviewing the crime scenes, including the hotel room, which would be littered with spent cartridge cases.
The duration of the bursts, as recorded, suggest that Mr. Paddock cared little about the militaryâs prescriptions for automatic fire. Sustained rapid fire is difficult to control and causes many weapons, especially light weapons, to overheat quickly.
Maj. Dave Eastburn, a Pentagon spokesman, said that the military had no record of Mr. Paddock serving in any of the uniformed services.
The length of the bursts also indicate that Mr. Paddock had magazines capable of holding scores of rounds, allowing him to fire longer without reloading.
Nevada, unlike some states, has no laws limiting ammunition magazine capacities.
The remaining limited details about how Mr. Paddock organized for the crime raise more questions. Two law enforcement officials said he used a hammer to break the windows through which he fired, and Sheriff Lombardo said he possessed scopes for at least some of his weapons, though it was not clear what roles they played.