Stephen Paddock, the gunman who killed 58 people and injured more than 500 in Las Vegas on Sunday, set up a number of cameras in and around his hotel suite.
Two cameras in the hallway and one in the peephole allowed him to see if "law enforcement or security" were approaching, police said.
Officers are still trying to determine why Paddock, 64, opened fire on a concert from the Mandalay Bay Hotel.
However, they do know there was a high degree of planning.
The authorities in Las Vegas revised the death toll down from 59 on Tuesday evening, saying that one of the bodies was that of the gunman.
Clark County Sheriff Joseph Lombardo told reporters: "This individual was pre-meditated. Obviously pre-meditated, the fact that he had the type of weaponry and the amount of weaponry in that room.
"It was pre-planned extensively and I'm pretty sure he evaluated everything he did in his actions."
Undersheriff Kevin McMahill suggested the attack may have stopped when Paddock was disturbed, shooting a security guard.
However, a senior US homeland security official speaking on condition of anonymity told Reuters there was "no evidence" Paddock had "mental illness or brain damage".
Police found 23 guns in Paddock's hotel room, as well as firearms and explosives at his home. In total, across three locations, 47 firearms had been recovered, officials said.
a former accountant with a big gambling habit, lived in a community of senior citizens in the small town of Mesquite, north-east of Las Vegas.
He reportedly shared his house there with Marilou Danley.
Police still consider Ms Danley, the woman thought to have been his girlfriend, "a person of interest", he said, adding they were "in conversation".
Ms Danley had been in the Philippines, but a Philippines immigration bureau spokeswoman told reporters the woman had flown out of the country and had now arrived in Los Angeles.
Police have also found no links to any foreign or domestic terrorist organisations. Paddock, who appears to have killed himself before police stormed his hotel room, had no criminal record and was not known to police.
The shooting - the worst in modern US history - has sparked debate over US gun laws, but President Donald Trump however said the discussion was "not for now".
After visiting Puerto Rico on Tuesday, he said "perhaps that [time] will come" for a debate. Earlier, he had said: "We'll be talking about gun laws as time goes by".
Mr Trump, whose position on gun control has changed over the years, gave no further detail. He also declined to call the attack domestic terrorism.