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.
Clark County Sheriff Joseph Lombardo said the cameras, including one on a service cart, appeared to be in place to monitor the police.
Police 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.
Sheriff 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.”
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.
Photos from the hotel room of guns used in the attack have been obtained by Boston 25 News.
Police still consider the woman thought to have been his girlfriend, Marilou Danley, “a person of interest”, he said. She is in the Philippines.
“We are in conversation,” Sheriff Lombardo said.
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.
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”.
So-called Islamic State claimed on Monday to be behind the attack, saying Paddock had converted to Islam some months ago. But the group provided no evidence for this and has made unsubstantiated claims in the past.
The FBI said it had found “no connection to an international terrorist organisation” (Courtesy BBC)