The girlfriend of the Las Vegas mass murderer has described him as a "kind, caring, quiet man" as police pored over the gunman's "secret life".

Marilou Danley was interviewed by FBI agents on Wednesday in a bid to discover the motive of Stephen Paddock, who is said to have spent decades acquiring weapons.

There was no evidence at this point to indicate that the mass shooting was terrorism, FBI Special Agent in Charge Aaron Rouse told a news conference, adding no one had been taken into custody as a suspected accomplice.

Ms Danley said in a statement that she had no clue he was planning the massacre that left 59 dead and more than 500 injured in America's worst ever mass shooting.