The Las Vegas gunman transferred $US100,000 overseas in the days before the attack and planned the massacre so meticulously he even set up cameras inside the peephole of his high-rise hotel room and on a service cart outside his door, apparently to spot anyone coming for him, authorities say.
Meanwhile investigators are taking a harder look at the shooter's girlfriend and what she might have known about Sunday night's (Monday AEDT) attack at a country music festival, with the sheriff naming her a "person of interest".
Authorities are trying to determine why Stephen Paddock killed 58 people in the deadliest mass shooting in modern US history.
They have been speaking with Filipina-Australian girlfriend Marilou Danley, 62, who was out the country at the time of the shooting.
She arrived in Los Angeles on Tuesday night (Wednesday AEDT) on a flight from Manila and was met by FBI agents. She will later be taken to Las Vegas.
Sheriff Joseph Lombardo said he was "absolutely" confident authorities would find out what set off Paddock, a 64-year-old high-stakes gambler and retired accountant who killed himself before police stormed the 32nd floor of his suite in the Mandalay Bay casino.
Paddock transferred $US100,000 ($A127,180) to the Philippines in the days before the shooting, a US official said.
A senior US homeland security official, who has been briefed regularly on the probe but spoke on condition of anonymity, said the working assumption of investigators was the money was intended as a form of life insurance payment for Danley.
The official said US authorities were eager to question Danley about whether Paddock encouraged her to leave the US before he went on his rampage.
Fresh details about the massacre and the arsenal Paddock amassed emerged on Tuesday.
Authorities released police body camera video that showed the chaos of the attack as officers tried to figure out the location of the shooter and shuttle people to safety.
Amid sirens and volleys of gunfire, people yelled, "They're shooting right at us" while officers shouted, "Go that way!"
Clark County Undersheriff Kevin McMahill said the shooting spanned between nine and 11 minutes.
The cameras Paddock set up at the Mandalay Bay casino were part of his extensive preparations, which included stockpiling nearly two-dozen guns in his room before opening fire on the concert below.
McMahill said the cameras included one in the peephole and two in the hallway.
"I anticipate he was looking for anybody coming to take him into custody," Lombardo said.
During the rampage, a hotel security guard who approached the room was shot through the door and wounded in the leg.
Forty-seven firearms were recovered from three locations searched by investigators - Paddock's hotel suite, his home in Mesquite and another property associated with him in Reno, Nevada, according to Jill Snyder, special agent for the US Bureau of Alcohol Tobacco and Firearms (ATF).
She said 12 of the guns found in the hotel room were fitted with so-called bump-stock devices that allow the guns to be fired virtually as automatic weapons.
The devices are legal under US law, even though fully automatic weapons are for the most part banned.
The rifles, shotguns and pistols were bought in four states - Nevada, Utah, California and Texas - Snyder told reporters at an Tuesday news conference.
A search of Paddock's car turned up a supply of ammonium nitrate, a fertiliser that can be formed into explosives.
Police also confirmed that photos widely published online showing the gunman's body, his hands in gloves, lying on the floor beside two firearms and spent shell casings, were authentic crime-scene images obtained by media outlets.
An internal investigation was underway to determine how they were leaked.
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