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.
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”.
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.
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.
What do we know of the gunman?
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”.
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”.
How did the attack unfold?
The final shows of the three-day Route 91 country music festival were in full swing when the gunman struck.
According to police, Paddock had booked into the hotel four days earlier, on 28 September, reportedly using some of Ms Danley’s identity documents.
Aside from the 23 guns found in the two-room hotel suite, Sheriff Lombardo said there were 10 suitcases.
Thousands were enjoying a performance by singer Jason Aldean when the first of several bursts of automatic gunfire rang out, starting at 22:08 local time (05:08 GMT on Monday).
Hundreds of shots were fired over the course of nine minutes, according to police.
Concert-goers scrambled for cover, flattening themselves against the ground, rushing for the exits or helping others to escape as Paddock sprayed the site from his high vantage point.
It is thought he moved between two windows in his suite as he carried out the attack.
A specialised Swat police team later stormed the suite to find Paddock shot dead.
Minutes before, a hotel security guard was injured when Paddock fired through the door.