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.
The shooter was not known to police.
President Donald Trump described the attack as "pure evil" in an address from the White House on Monday.
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