Las Vegas: Police believe Las Vegas gunman Stephen Paddock may have had an accomplice as he prepared for the deadliest massacre in modern US history, amassing guns for more than a year and leading what has been described as "a secret life".

His Australian girlfriend Marilou Danley, who has been with FBI agents since returning to the US from the Philippines on Wednesday, said she saw no signs that her "kind, caring, quiet" partner was planning such an act.

Ms Danley, 62, said she thought her partner was trying to break up with her when he bought her a plane ticket to the Philippines two weeks ago and then wired her $US100,000 ($127,000) for her to buy a house there.

"I loved him and hoped for a quiet future together with him. He never said anything to me or took any action that I was aware of that I understood in any way to be a warning that something horrible like this was going to happen," she said in a statement released on Thursday.