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.
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”.
He earlier described Paddock as “a sick man, a demented man”.
But a senior US homeland security official, speaking on condition of anonymity to news agency Reuters, said there was “no evidence” of “mental illness or brain damage”.
Nor have police found 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.
However, 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 have been recovered, officials said.
Police still consider the woman thought to have been his girlfriend, Marilou Danley, “a person of interest”, he said, adding they were “in conversation”.
Ms Danley had been in the Philippines, but is now on her way back to the US, the Philippine immigration bureau spokeswoman told reporters on Wednesday.
The shooting has prompted calls for reform to US gun laws.
But Mr Trump – who has been backed by the National Rifle Association, and spoke often of protecting the Second Amendment during his campaign – has tried to steer clear of leaning too far either way.
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.
Mr Trump also declined to call the attack domestic terrorism.
Paddock, 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. Initially police had said she was in Japan rather than the Philippines and discounted her involvement.
Nineteen firearms, some explosives and several thousand rounds of ammunition with electronic devices were found at the property.
Officers also found ammonium nitrate in Paddock’s car. The chemical compound used in fertilisers can be a component of bombs such as that deployed in the 1995 Oklahoma City attack.
David Famiglietti of the New Frontier Armory told the BBC that Paddock had purchased firearms at his store in North Las Vegas in the spring of this year, meeting all state and federal requirements, including an FBI background check.
However, the shotgun and rifle Paddock bought would not have been “capable of what we’ve seen and heard in the video without modification”, Mr Famiglietti said.
The fast shooting rate audible in recordings of Sunday night’s attack indicates that Paddock may have modified his guns with legal accessories to make them fire at speeds approaching those of automatic weapons.
Despite the large cache of weapons found in the killer’s home, his brother, Eric, is struggling to accept that he acted in this way. He said he was “in shock, horrified, completely dumbfounded”. -BBC