Stephen Paddock appeared to be settling into a quiet life two years ago when the wealthy 64-year-old apartment manager and high-stakes gambler bought a home in a rural Nevada retirement community near Las Vegas.
But over time he amassed an arsenal of weapons prior to committing the worst mass shooting in recent U.S. history. There were 23 guns found in his Mandalay Bay room, roughly an hour's drive southwest from his home.
Two officials familiar with the investigation told the AP that Paddock had two bump stocks, which can convert semi-automatic firearms into fully automatic ones. They are investigating whether those stocks were used to modify weapons used in the massacre, according to the officials, who were briefed by law enforcement and spoke on condition of anonymity because the investigation is still unfolding.
A semi-automatic weapon requires one trigger pull for each round fired. With a fully automatic firearm, one trigger pull can unleash continuous rounds until the magazine is empty.
The device basically replaces the gun's shoulder rest, with a "support step" that covers the trigger opening. By holding the pistol grip with one hand and pushing forward on the barrel with the other, the shooter's finger comes in contact with the trigger. The recoil causes the gun to buck back and forth, "bumping" the trigger.
The weapons can be expensive, but Eric Paddock, the killer's brother, said Stephen was a multimillionaire who made much of his money investing in real estate. He was not aware of his brother having any recent financial difficulties.
In recent weeks, he made gambling transactions worth tens of thousands of dollars, although it was unclear whether they were wins or losses, NBC News reported, citing unidentified law enforcement officials.
And he may not aroused suspicion purchasing weapons. Las Vegas police say he had no criminal record beyond a traffic violation.
In early 2015, Paddock bought a modest three-bedroom home in a new housing development for retirees on the dusty edge of Mesquite, a small desert town popular with golfers and gamblers, that straddles Nevada's border with Arizona.
He paid about $370,000 US, according to property records.
Authorities searched that home, as well as another property he owned in Reno, Nev. Additional weapons were found, and it appears Paddock amassed his stock through purchases in multiple states.
The owner of a gun shop in Mesquite confirmed Paddock bought firearms there and never gave any indication that he might have been unstable.
Guns & Guitars general manager Christopher Sullivan said in a statement Monday that Paddock showed no signs of being unfit to buy guns.
The store said they are cooperating with authorities, with store spokesman Shawn Vincent declining comment on how many guns Paddock bought there, saying those details could only be shared with authorities.
The owner of a Utah gun store says Paddock visited the store several times this year and bought a shotgun after passing a federal gun background check.
Dixie GunWorx owner Chris Michel says Paddock said that he was new to the area and was visiting local gun shops.
Paddock bought the shotgun in February and last visited the store in St. George, Utah, in the spring. It's a 40-minute drive from where he lived in Mesquite, Nev.
Michel told The Associated Press he chatted with Paddock to get to know him and make sure there were no signs that he should not be allowed to buy a gun.
"There were no red flags," Michel said. "I had no idea he would be capable of this."
Eric Paddock was among those baffled as to why his brother holed up in a Vegas Strip hotel, using a hammer to break a the room's windows and acting as a sniper as unsuspecting tourists and revellers scurried or fell to the ground.
"He was a wealthy guy and he liked to play video poker and he liked to go on cruises," Eric Paddock, told reporters from his doorstep in Orlando, Fla., on Monday, the day after the shooting.
"He's never drawn his gun, it makes no sense," Eric Paddock said. He said he was aware that his brother had a couple of handguns he kept in a safe, perhaps a long rifle, but no automatic weapons to his knowledge.
"We are completely dumbfounded. We can't understand what happened," he told the Orlando Sentinel.
The two were last in touch in early September, exchanging text messages about power outages after Hurricane Irma slammed into Florida, where their 90-year-old mother still lives. Stephen had recently sent a walker by mail to his mother.
Public records point to an itinerant existence before Paddock ultimately settled in Mesquite. He lived in Florida, Texas and California, as well as a few years in other parts of Nevada.
Paddock had a hunting licence in Texas, where he lived for a while and managed an apartment complex. He got his pilot's licence, and had at least one single-engine aircraft registered in his name.
Lockheed Martin said that he worked for a predecessor of the company from 1985 to 1988, but offered no other details. A Lockheed Martin spokesperson said officials there were co-operating with authorities.
While Paddock managed to stay clear of the law, it was revealed Monday his father was Patrick Benjamin Paddock, a violent bank robber who was on the Federal Bureau of Investigation's Most Wanted list in the 1960s.
Eric Paddock said their father was not a presence in the boys' lives.
Police and public records show Stephen Paddock lived with a 62-year-old woman at his Mesquite home. Authorities said she had no connection with the attack and was out of the country at the time of the shooting.