But what led Stephen Paddock, 64, to unleash the carnage remains largely a mystery.
"What we know is that Stephen Paddock is a man who spent decades acquiring weapons and ammo and living a secret life, much of which will never be fully understood," Clark County Sheriff Joseph Lombardo told reporters on Wednesday night.
Lombardo said he found it hard to believe that the arsenal of weapons, ammunition and explosives recovered by police in their investigation could have been assembled by Paddock completely on his own.
"You have to make an assumption that he had some help at some point," Lombardo said.
Some 489 people were also injured when Paddock strafed an outdoor concert with gunfire on Sunday night from his 32nd-floor suite of the Mandalay Bay hotel on the Las Vegas Strip. He then took his own life.
Police recovered nearly 50 firearms from three locations they searched, nearly half of them from the hotel suite. Officials said 12 of the rifles there were fitted with bump stocks, allowing the guns to be fired almost as though they were automatic weapons.
Like other recent mass shootings, the incident stirred the debate in Washington over regulating firearm ownership, which is protected by the Second Amendment of the U.S. Constitution.
Republicans, who currently control the White House and both chambers of Congress, have fought off Democratic calls for stricter background checks or federal limits on magazine size following past mass shootings.
But congressional Republicans said they would be willing to investigate the bump stocks that allow legal semi-automatic rifles to behave similarly to fully automatic weapons, which are largely illegal in the United States.
Investigators were examining the possibility Paddock's purchase of more than 30 guns in October 2016 may have been precipitated by some event in his life, Lombardo said.
The FBI said on Wednesday there remained no evidence indicating that the shooting spree was an act of terrorism.
Paddock's girlfriend, Marilou Danley, was questioned by the FBI on Wednesday and said in a statement she had been unaware of Paddock's plans.
"He never said anything to me or took any action that I was aware of that I understood in any way to be a warning that something horrible like this was going to happen," Danley, 62, said in the statement released by her lawyer, Matt Lombard.
Danley, who returned late Tuesday from a family visit to the Philippines, is regarded by investigators as a "person of interest." Lombard said his client was cooperating fully with authorities.
A Federal Bureau of Investigation official in Las Vegas, meanwhile, said no one has been taken into custody.
An Australian citizen of Filipino heritage, Danley said she flew back to the United States voluntarily "because I know that the FBI and Las Vegas Police Department wanted to talk to me, and I wanted to talk to them."
Danley shared Paddock's home at a retirement community in Mesquite, Nevada, northeast of Las Vegas, before traveling to the Philippines in mid-September.
Investigators questioned her about Paddock's weapons purchases, a US$100,000 wire transfer to a Philippine bank that appeared to be intended for her, and whether she saw any changes in his behavior before she left the United States.
Danley said Paddock had bought her an airline ticket to visit her family and wired her money to purchase property there, leading her to worry he might be planning to break up with her.
Paddock's brother Eric told reporters the US$100,000 transfer was evidence that "Steve took care of the people he loved," and that he probably wanted to protect Danley by sending her overseas before the attack.
Discerning Paddock's motive has proven especially baffling as he had no criminal record, no known history of mental illness and no outward signs of social disaffection, political discontent or extremist ideology, police said.
