Marilou Danley says her boyfriend displayed signs of mental health issues and would moan and scream "Oh, my God" in his sleep ahead of his sniper attack on concert goers in Las Vegas, according to two former FBI officials.
Ms Danley, an Australian citizen, was interviewed by FBI agents in Los Angeles on Wednesday about Stephen Paddock, who killed 58 people and injured almost 500 others on the weekend.
"She said he would lie in bed, just moaning and screaming, 'Oh, my God'," one of the former FBI officials told NBC News.
Ms Danley also spoke about Paddock displaying "mental health symptoms", according to the other former FBI official.
The FBI, Las Vegas Police and other agencies, including Australian law enforcement, are working to unearth why Paddock meticulously planned Sunday's attack from a luxury suite in the Mandalay Bay hotel.
A clear motive for the actions of the 64-year-old professional gambler and property investor, who stockpiled a large quantity of guns and explosives over the past 12 months, has yet to be identified.
Investigators do not believe Paddock's mental health had deteriorated to a point that it triggered his horrific shooting spree.
Paddock had reportedly been prescribed anti-anxiety medication Valium.
Las Vegas Sheriff Joe Lombardo has raised the possibility that Paddock may not have been working alone in executing America's worst modern-day shooting spree.
MSNBC reported on Friday a mobile phone charger that did not match Paddock's phone was found in his Mandalay Bay suite and it appears the keycard for his room was used to open the door when his car had left the casino car park.
Ms Danley, a 62-year-old Philippines-born grandmother who worked as a high-limit hostess at Reno, Nevada's Atlantis casino, was in her homeland when Paddock launched the attack.
She flew to LA on Tuesday and in a statement released by her lawyer after the FBI interview, said Paddock "never said anything to me or took any action that I was aware of that I understood in any way to be a warning something horrible like this was going to happen".
Ms Danley claimed Paddock surprised her with a cheap air ticket a fortnight earlier to fly to the Philippines to visit family and wired her money - reportedly $US100,000 - to buy a house in the Philippines.
Ms Danley said she believed it was Paddock's way of breaking up with her.
Australian Federal Police Commissioner Andrew Colvin said Australian authorities were "working very closely with our partners in the US" to help probe the Las Vegas massacre.