The Las Vegas attacker transferred $100,000 overseas in the days before he killed 59 people, and planned the massacre so meticulously that 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 have said.
Meanwhile, investigators are taking a harder look at the attacker's girlfriend and what she might have known about the assault on a country music festival, with the sheriff naming her a "person of interest".
The girlfriend, Marilou Danley, 62, returned to the United States from the Philippines on Tuesday night and was met at Los Angeles International Airport by FBI agents, according to a law enforcement official.
The official was not authorised to publicly discuss the matter and spoke to The Associated Press news agency on condition of anonymity.
Authorities are trying to determine why Stephen Paddock killed 59 people in the deadliest mass shooting in modern US history.
They have been speaking with Danley, who was out the country at the time of the shooting, and "we anticipate some information from her shortly," Sheriff Joseph Lombardo said hours before she arrived.
Lombardo said he is "absolutely" confident authorities will find out what set off Paddock, a 64-year-old high-stakes gambler and retired accountant who killed himself before police stormed his 32nd-floor room.
Authorities released police body camera video that showed the chaos of the attack as officers tried to figure out the location of the attacker 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.
Paddock transferred $100,000 to the Philippines in the days before the shooting, a US official briefed by law enforcement but not authorised to speak publicly because of the continuing investigation told The Associated Press news agency.
Investigators are still trying to trace that money and also looking into a least a dozen financial reports over the past several weeks that said Paddock gambled more than $10,000 a day, the official said.
Cameras set up
The cameras Paddock set up at the Mandalay Bay hotel-casino were part of his extensive preparations that 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 Sunday night rampage, a hotel security guard who approached the room was shot through the door and wounded in the leg.
"The fact that he had the type of weaponry and amount of weaponry in that room, it was preplanned extensively," the sheriff said, "and I'm pretty sure he evaluated everything that he did and his actions, which is troublesome."
Lombardo said the investigation is proceeding cautiously in case criminal charges are warranted against someone else.
"This investigation is not ended with the demise of Mr Paddock," the sheriff said. "Did this person get radicalised unbeknownst to us? And we want to identify that source."
In addition to the cameras, investigators found a computer and 23 guns with him at the hotel, along with 12 "bump stock" devices that can enable a rifle to fire continuously, like an automatic weapon, authorities said.
Nineteen more guns were found at Paddock's Mesquite home and seven at his Reno house.
Video shot outside the broken door of the room shows an assault-style rifle with a scope on a bipod. The sheriff said an internal investigation has been launched to find out how that footage was obtained.
Some investigators turned their focus on Tuesday from the Paddock's perch to the festival grounds where his victims fell.
A dozen investigators, most in FBI jackets and all wearing blue booties to avoid contaminating the scene, documented evidence at the site where gunfire rained down and country music gave way to screams of pain and terror.
"Shoes, baby strollers, chairs, sunglasses, purses. The whole field was just littered with things," said Nevada Attorney General Adam Laxalt after touring the site on Monday. "There were bloodstains everywhere."
More than 500 people were injured in the rampage, some by gunfire, some during the chaotic escape. At least 45 patients at two hospitals remained in critical condition.
All but three of the dead had been identified by Tuesday afternoon, Lombardo said.
Source: AP news agency