Stephen Paddock, the gunman who killed 59 people and injured more than 500 in Las Vegas on Sunday, set up a number of cameras in and around his hotel suite, police said.
Two cameras in the hallway and one in the peephole allowed him to see if "law enforcement or security" were approaching, the BBC reported.
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.
Clark County Sheriff Joseph Lombardo told reporters on Tuesday: "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.
The shooting - the worst in modern US history - has sparked debate over US gun laws, but President Donald Trump has said the discussion over what, if anything, needs to be done was "not for now", the BBC report said.
Paddock had stockpiled a total of 42 firearms in his 32nd floor hotel room and at his home, 130 km away from the site of the massacre, officials said.
Authorities found 19 weapons from his home in Mesquite, a small city in Nevada, and 23 in the Mandalay Bay Hotel room in Las Vegas from where Paddock opened fire on 22,000 people attending a concert at a country music festival on Sunday night.
Police found in the gunman's car several kilos of ammonium nitrate, a substance used in the manufacture of explosives.
The shooter spent his last moments firing desperately at the police through the door of his room at the hotel, said Lombardo.
With hundreds of victims still hospitalised, officials said the death toll was likely to rise.
--IANS
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