Stephen Paddock's girlfriend recalls the Las Vegas shooter lying in bed screaming to himself, reports claim.

Investigators believe the shooter may have been in “physical or mental anguish,” a former FBI official who was briefed on the investigation told NBC News. Another said the shooter displayed "mental health symptoms”.

"[His girlfriend] said he would lie in bed, just moaning and screaming, 'Oh, my God,'" one of the officials recalled.

Paddock was prescribed an anti-anxiety drug in June. Employees at a Starbucks near his Reno home recall him berating his girlfriend in public. But investigators do not believe his mental health was poor enough to have triggered an attack of this scale, the officials told NBC.

Last weekend, Paddock opened fire on a country music concert from his 32nd-story hotel window, shooting more than 500 people before killing himself. The new details about his mental state may provide hints as to the motive for the shooting, which police are still struggling to determine.

Las Vegas shooting – in pictures Show all 15 1 /15 Las Vegas shooting – in pictures Las Vegas shooting – in pictures People scramble for shelter at the Route 91 Harvest country music festival after gun fire was heard Getty Las Vegas shooting – in pictures People carry a person at the Route 91 Harvest country music festival after shots were fired David Becker/Getty Las Vegas shooting – in pictures People run from the Route 91 Harvest country music festival after gun fire was heard David Becker/Getty Las Vegas shooting – in pictures A handout photo released via Twitter by Eiki Hrafnsson (@EirikurH) showing concertgoers running away from the scene (C) after shots range out at the Route 91 Harvest festival on Las Vegas Boulevard EPA/Eiki Hrafnsson Las Vegas shooting – in pictures People lie on the ground at the Route 91 Harvest country music festival after hearing gun fire Getty Las Vegas shooting – in pictures A man in a wheelchair is taken away from the Route 91 Harvest country music festival after hearing gun fire David Becker/Getty Las Vegas shooting – in pictures People stand on the street outside the Mandalay Bay hotel near the scene of the Route 91 Harvest festival on Las Vegas Boulevard EPA/Paul Buck Las Vegas shooting – in pictures FBI agents confer in front of the Tropicana hotel-casino after a mass shooting during a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Las Vegas police run by a banner on the fence at the Route 91 Harvest country music festival grounds after shots were fired David Becker/Getty Las Vegas shooting – in pictures An injured person is tended to in the intersection of Tropicana Ave. and Las Vegas Boulevard after a mass shooting at a country music festival Ethan Miller/Getty Las Vegas shooting – in pictures Metro Police officers pass by the front of the Tropicana hotel-casino after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures A woman sits on a curb at the scene of a shooting outside of a music festival along the Las Vegas Strip AP/John Locher Las Vegas shooting – in pictures A cowboy hat lays in the street after shots were fired near a country music festival in Las Vegas Getty Las Vegas shooting – in pictures Las Vegas Metro Police and medical workers stage in the intersection of Tropicana Avenue and Las Vegas Boulevard South after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Sheriff Joe Lombardo (2-R) speaking during a press briefing in the aftermath of the active shooter incident on Las Vegas Boulevard EPA

“In the spirit of the safety of this community or anywhere else in the United States I think it’s important to provide that information, but I don’t have it,” Clark County Sheriff Joseph Lombardo said in an interview on Thursday. “We don’t know it yet.”

Police had hoped Paddock’s girlfriend, Marilou Danley, would be able to provide some insight. But Ms Danely told investigators she knew nothing about Paddock’s plans for the attack. Through an attorney, she described Paddock as a “kind, caring, 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,” she said.

Partner of Las Vegas gunman releases statement describing him as "kind, caring, quiet man"

Paddock’s brother, Eric Paddock, was similarly confused.

“Something had to happen to Steve – I'm not even trying to excuse it – but something happened that drove him into the pit of hell and he did this,” he told CBS.

A man on the Las Vegas Strip films on his phone the two broken windows of the Mandalay Bay hotel from which killer Stephen Paddock let loose (ROBYN BECK/AFP/Getty Images)

Investigators have discovered a note in Paddock’s hotel room that they are still analysing for significance. They do not believe it was a manifest or a suicide note, according to Mr Lombardo.

Authorities told NBC that Paddock may have also scouted other locations in Chicago and Boston before his attack. A Chicago hotel overlooking popular music festival Lollapalooza confirmed to The Independent that someone named Stephen Paddock had booked a room during the festival, but failed to show up.

For now, however, authorities are still at a loss for what could have motivated his killing spree. Answering reporters' questions on Monday, Mr Lombardo seemed frustrated.