The gunman, who fired from the 32nd floor of the Mandalay Bay Hotel on the Las Vegas strip, was shot and killed by police.
Witnesses say hundreds of shots were fired and the sound of automatic gunfire was heard.
Police are trying to locate an Asian female named as Marilou Danely who they say was travelling with the gunman before the shooting.
Sheriff Joe Lombardo said it was a "lone wolf" style attack.
Police say reports of other incidents at different locations along the Las Vegas strip are false.