The attack came at the Route 91 Harvest Festival near the Mandalay Bay Resort and Casino in Las Vegas.
Paddock, 64, of Mesquite, Nevada, had one camera mounted in his hotel room, apparently to record himself, police told ABC News.
Interstate 15 was briefly closed, and flights at McCarran International Airport were suspended.
"We're trying to understand what happened", his brother said. Twelve of the rifles there were fitted with so-called bump stocks, officials said, allowing the guns to be fired almost as though they were automatic weapons. "He's never hit anybody", Eric Paddock said.
"I will never forget the sound of that gunfire", Nashville, Tennessee, publicist Karen Gale, 44, said.
The Islamic State militant group claimed responsibility for the massacre, but US officials said there was no evidence to support the claim.
Panic and chaos took over the Las Vegas strip as Paddock opened fire and hundreds of rounds rained down from the 32 floor of the hotel.
After a pause, the gunman fired another volley, with the muzzle flashes visible from the casino as victims fell to the ground, while others fled in panic. "It didn't stop", said witness Rachel de Kerf.
An apparent gun enthusiast, Paddock was familiar to local gun owners.
"We believe Paddock is solely responsible for this heinous act", Clark County Assistant Sheriff Todd Fasulo said in a press conference late Monday.
The big question for Segreti was how do you keep guns away from people who are mentally ill but still allow access to them for law-abiding citizens with no mental illnesses.
In his Mesquite home police found the explosive tannerite, which can be bought commercially.
Police also discovered cameras that Paddock had set up inside and outside of his hotel room, and they are evaluating whether there's footage of the shooting.
The purchasing of fully automatic weapons has been significantly restricted in the US since the 1930s.
Neighbors described Paddock as a loner, and that Ms. Danley was his girlfriend. "And it always has".
"We went back because I'm a nurse and I felt I had to", she told CNN affiliate KTNV. "That's the one in this one, and we are not there yet", FBI Deputy Director Andrew McCabe said.
In addition, federal authorities were investigating Paddock's wiring of $100,000 last week to the Philippines, the home country of his girlfriend.
In early August, Paddock booked a room at Chicago's 21-story Blackstone Hotel that overlooked the park where the Lollapalooza music festival was being held, a law enforcement official said Thursday.
U.S. Rep. John Faso, R-19, also spoke out about the tragedy.
President Donald Trump, who visited Las Vegas victims, first responders and heroes on Wednesday, offered his own analysis of the shooter's motives.
"Stephen Paddock worked for a predecessor company of Lockheed Martin from 1985 until 1988", the company said. Authorities found weapons and ammunition but could not go into details, according the Mesquite Police Department.
The 2016 Pulse nightclub shooting in Orlando, Florida, previously was the deadliest, with 49 killed. We mourn a wonderful member of the Disney family: "Carrie Barnette", tweeted Disney Chairman and CEO Robert Iger.
Police had no prior knowledge of the gunman before the attack, Sheriff Lombardo said. "It hurts my heart that this would happen to anyone who was just coming out to enjoy what should have been a fun night", Aldean wrote Sunday night.