The Las Vegas gunman set up cameras inside and outside his hotel room before firing for at least nine minutes at a concert crowd below.
Police spoke of how Stephen Paddock carefully planned his actions before he opened fire on concertgoers at a “progressively rapid rate” through windows on the 32nd floor of the Mandalay Bay Hotel and Casino.
He placed one camera in a food service cart in the corridor, while a second camera was placed outside his room and a third in the peep hole of his two-room suite, officials said.
Investigators who searched his room also found a computer and 12 “bump stock” devices that can enable a rifle to fire continuously, like an automatic weapon.
Police said the shooting continued for between nine and 11 minutes, between 10.08pm and 10.19pm on Sunday.
Paddock had amassed an arsenal of at least 42 guns, explosives and several thousand rounds of ammunition, police said.
He used at least 10 suitcases to smuggle the weapons into his room after checking in on Thursday last week.
Images posted by US media showed automatic weapons attached to tripods lying on the ground of the hotel room, with scores of bullet casings littering the floor around them.
At a news conference, Clark County Sheriff Joe Lombardo said: “The fact that he had the type of weaponry and amount of weaponry in that room, it was pre-planned extensively.”
“Bump stock” devices are legal under US law, even though fully automatic weapons are for the most part banned.
California senator Dianne Feinstein, who has raised concerns over the sale of such accessories, said on Tuesday that Paddock had modified at least one of his weapons.
She said: “Individuals are able to purchase bump fire stocks for less than $200 (£150) and easily convert a semi-automatic weapon into a firearm that can shoot between 400 and 800 rounds per minute and inflict absolute carnage.”
Police believe the gunman, who killed 59 people and injured hundreds more, checked into the hotel on Thursday, carrying at least 23 guns, including an assault rifle with a scope on a bipod.
During his killing spree, Paddock shot a security guard who approached the room in the leg.
The 64-year-old retired accountant killed himself before he could be detained.
He added that what police had to determine now was whether anyone else was involved.
Among the persons of interest was Paddock’s girlfriend, Philippines-born Marilou Danley, who arrived in the US early on Wednesday morning for questioning.
Sheriff Lombardo said officers discovered five handguns, two shotguns and a “plethora” of ammunition in the Reno property.
Officers have also found several pounds of ammonium nitrate, a chemical used to make explosives, in his car at the hotel.
Among the wounded were 12 off-duty firefighters, including two who were hit while they tended to gunshot victims, union officials said.