The girlfriend of the Las Vegas gunman has said she had no clue that her partner was planning the violent attack that left at least 59 people dead and more than 500 wounded on Sunday.

Marilou Danley issued the statement on Wednesday following her return to the United States from her native Philippines where she was for more than two weeks.

After being questioned for much of the day by FBI agents, Danley, 62, said she loved her boyfriend, Stephen Paddock, and knew him as a kind, caring and quiet man.

"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."

READ MORE: Las Vegas gunman transferred $100K, set up cameras at hotel

She said Paddock had suggested she travel to the Philippines after he found her a cheap airline ticket.

Danley said she was initially pleased when Paddock wired her money in the Philippines to buy a house for her family, but she later feared it was a way to break up with her.

"It never occurred to me in any whatsoever that he was planning violence against anyone," Danley, 62, said in the statement read by her lawyer, Matthew Lombard, outside FBI headquarters in Los Angeles.

Danley said she was devastated by the attack on concertgoers - the worst mass shooting in modern US history - and her prayers went out to all the victims.

"I am a mother and grandmother and my heart breaks for all who have lost loved ones," she said.

Danley said she returned to the US voluntarily and will cooperate fully with the investigation. Police say she could be key to understanding Paddock's motive.

Hunt for clues

Investigators are busy reconstructing the attacker's life, behaviour and the people he encountered in the weeks leading up to attack, FBI Deputy Director Andrew McCabe said. That includes examining his computer and mobile phone.

But as of Wednesday, investigators were unable to explain what led Paddock to fire down on a country music festival from the windows of his 32nd-floor room at the Mandalay Bay hotel-casino. He killed himself as police closed in.

"This individual and this attack didn't leave the sort of immediately accessible thumbprints that you find on some mass casualty attacks," McCabe said.

READ MORE: Who is Stephen Paddock?

The 64-year-old retired accountant quietly stockpiled an arsenal of high-powered weapons while pursuing a passion for high-stakes gambling at Nevada casinos, where his game of choice was video poker, a relatively solitary pursuit with no dealer and no humans to play against.

Neighbours described Paddock as friendly, but he wasn't close to them.

"He was a private guy. That's why you can't find out anything about him," his brother, Eric Paddock, said from his home in Florida. As for what triggered the attack, the brother said: "Something happened that drove him into the pit of hell."

Occasionally, Paddock shared news of his gambling winnings, his brother said, recalling a photo text message he received showing a $40,000 payout.

It was in a casino where Paddock met his girlfriend, who was a high-limit hostess for Club Paradise at the Atlantis Casino Resort Spa in Reno, Eric Paddock told The Washington Post.

Paddock wired $100,000 to the Philippines days before the shooting, a US official not authorised to speak publicly because of the continuing investigation said on condition of anonymity. Investigators are trying to trace that money.

Paddock had no known criminal history. Public records contained no indication of any financial problems, and his brother described him as a wealthy real estate investor.

'Endless sorrow'

US President Donald Trump and first lady Melania Trump met privately with victims at a Las Vegas hospital on Wednesday and then with police officers and dispatchers, praising them and the doctors who treated the wounded.

He did not address those grieving until the end of his visit, when he called it a "very sad day for me personally".

"Our souls are stricken with grief for every American who lost a husband or a wife, a mother or a father, a son or a daughter," he said.

"We know that your sorrow feels endless. We stand together to help you carry your pain."

Paddock had stockpiled 47 guns since 1982 and bought 33 of them, mostly rifles, over the past year alone, right up until three days before the attack, Jill Snyder, an agent with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, told CBS on Wednesday.