WASHINGTON (REUTERS) - The FBI on Wednesday (Oct 4) questioned the girlfriend of the gunman who slaughtered 58 concertgoers in Las Vegas, hoping to discover what motivated Stephen Paddock to open fire on a country music festival in the deadliest mass shooting in recent US history.

Marilou Danley, who returned to the United States late on Tuesday, is a "person of interest" in the investigation, Las Vegas police have said. Her relatives in the Philippines told reporters she had a "clean conscience" and no prior knowledge of the attack.

More than 500 people were injured, some trampled in the pandemonium, when Paddock, 64, fired bullets from his 32nd-floor hotel room for about 10 minutes on Sunday night. He killed himself before police stormed his room, where they found as many as 23 guns, bringing the total death toll to 59.

Twelve of his rifles were fitted with so-called bump stocks, officials said, allowing the guns to be fired almost as though they were automatic weapons.

US President Donald Trump visited Las Vegas on Wednesday to pay his respects and support first responders, marking the first time he has had to deal as president with a major mass shooting of the type that have killed hundreds of people in recent years in the United States.

Investigators have focused on Paddock's girlfriend Danley, 62, who once lived with Paddock and left the United States for the Philippines in September.

The Federal Bureau of Investigation met her plane from Manila at Los Angeles International Airport and took her away for questioning, said two US officials briefed on the case.

As of midday on Wednesday, there was no indication she was aware of Paddock's plans, the officials said.

They were questioning her about his weapons purchases, a US$100,000 (S$136,000) wire transfer to a Philippine bank that they think may have been intended for her, and about whether she saw any changes in his behaviour before she left the United States.

"Assuming she had no role in his actions, the most important thing is any light she can shed on Paddock's motive," said the official, who spoke about the continuing investigation only on condition of anonymity.

Danley, who has retained Los Angeles-area defense lawyer Matthew Lombard, had assured her family she has a "clean conscience," her brother told ABC News in the Philippines.

"I called her up immediately and she said, 'Relax, we shouldn't worry about it. I'll fix it. Do not panic. I have a clean conscience,'" Reynaldo Bustos told ABC in Manila.

Paddock's brother Eric told reporters the US$100,000 transfer was evidence that "Steve took care of the people he loved," and that he likely wanted to protect Danley by sending her overseas before the attack.

"He manipulated her to be completely as far away from this and safe when he did this," Eric Paddock said on Tuesday.

She arrived in Manila on Sept 15, flew to Hong Kong on Sept 22 and returned to Manila on Sept 25. She was there until she flew to Los Angeles on Tuesday night, according to a Philippine immigration official.

While Danley has not been arrested nor identified as a suspect, one expert noted that that was also the case for the wife of the shooter who killed 49 people at the Pulse nightclub in Orlando last year, previously the deadliest shooting spree in modern US history. Pulse shooter Omar Mateen was killed in a shootout with police.

Mateen's wife, Noor Salman, cooperated with investigators but was later indicted for aiding and abetting her late husband in support of the Islamic State in Iraq and Syria. Salman has pleaded not guilty.

"She cooperated her way right into a criminal indictment,"said Los Angeles criminal defence attorney Mark Geragos.

However, Geragos said, "Mere presence is not necessarily something that's going to get you indicted in most jurisdictions."

Or, as New York attorney Paul Shechtman noted, "Sadly, in America it's not a crime to know that your boyfriend has an arsenal of assault weapons."

Trump, touring a hospital in Las Vegas, told reporters Paddock was "very demented" but that investigators still had not nailed down a motive for the shooting.

"We struggle to explain to our children how such evil can exist," he said later at Las Vegas police command centre.

Meeting with law enforcement officers who rushed toward the shooter, Trump asked the head of the Las Vegas Metropolitan Police Department if investigators were any closer to establishing a motive.

"We've had a couple good leads and we're working our way through it," Sheriff Joseph Lombardo told the President.

Paddock had no criminal record, no known history of mental illness and no outward signs of social disaffection, political discontent or extremist ideology, police said earlier.

"We are not there yet," FBI deputy director Andrew McCabe said on Wednesday, speaking at the Cambridge Cyber Summit in Boston. "We have a lot to do."

However, Paddock appeared to be "descending into madness" in the months before the shooting, ABC News reported on Wednesday, citing an unnamed person briefed on the investigation.

Paddock had significant weight loss, an increasingly slovenly appearance and was obsessed with his girlfriend's former husband, ABC said.

In June, he was prescribed the anti-anxiety drug diazepam, commonly known as Valium, which can lead to aggressive behaviour, the Las Vegas Review-Journal reported, citing records from the Nevada Prescription Monitoring Programme.

Reuters was not able immediately to confirm the two reports.

"He was a private guy," Eric Paddock said of his brother."That's why you can't find out anything about him. Is he such a weirdo because he didn't have a Facebook account and post 50,000 pictures of himself every day?"

Paddock's case is extremely rare if not unique in that he left behind no clues to his motives, said Craig Jackson, a psychology professor at Birmingham City University in Britain who has studied spree killers for the past 10 years.

"We usually find something there in the background where they see themselves to be the victims. No one has listened to them and they feel this is the last recourse," Jackson said, adding that spree killers typically leave behind a manifesto or video diary.

"If there is something, I think we would have found it by now," Jackson said.