Gunman Stephen Paddock, 64, opened fire from the 32nd floor of the Mandalay Bay Hotel towards an open-air music festival on Sunday evening.
Police found 23 guns in his hotel room, as well as "in excess of" 19 firearms and explosives at his Nevada home.
But as yet, no clear reason for the killing has emerged.
Investigators have found no link to international terrorism, despite a claim from so-called Islamic State. Some investigators have suggested psychological issues, but there is no confirmation of this.