The 64-year-old, of Mesquite, Nevada, sprayed bullets at music-lovers as a gig by country singer Jason Aldean was ending on Sunday night, said officers.
The suspect opened fire from a room on the 32nd floor of the Mandalay Bay Hotel and Casino.
Paddock killed himself at the scene as officers closed in.
It is the deadliest shooting in modern US history, with the death toll surpassing the 49 killed at a nightclub in Orlando, Florida, in June 2016.
Las Vegas Sheriff Joseph Lombardo said investigators found "in excess of 10 rifles" in the hotel room that he had checked into on 28 September.
"We have no idea what his belief system was. Right now, we believe he was the sole aggressor and the scene is static," Sheriff Lombardo said, adding that the attack was not related to terrorism.
He described the suspect as a "distraught person intending to cause mass casualties", adding that he killed himself as police approached the hotel room.
Audio recorded at the concert suggests that Paddock used an automatic rifle in the attack on the crowd of 22,000 people.
He had no prior "derogatory" interactions with police, said Sheriff Lombardo, except for one citation several years ago which the sheriff declined to describe.