Tourists coming to gamble and party on the Strip will soon find something other than bright lights welcoming them to “Fabulous Las Vegas.”

Billboards will serve as a stark reminder that investigators remain stumped about what drove a gunman to mow down concert-goers from a perch in a high-rise casino hotel last Sunday.

“We still do not have a clear motive or reason why,” a frustrated Clark County Undersheriff Kevin McMahill said Friday. “We have looked at literally everything.”

Las Vegas Police clueless to find a motive for mass shooting

Investigators have chased 1,000 leads and examined Stephen Paddock's politics, his finances, any possible radicalisation and his social behaviour — typical investigative avenues that have helped uncover the motive in past shootings.

“We have been down each and every one of these paths,” McMahill said. “We all want answers.”

The FBI announced that billboards would go up around the city asking anyone with information to dial 800-CALL-FBI.

“If you know something, say something,” said Aaron Rouse, agent in charge of the Las Vegas FBI office. “We will not stop until we have the truth.”

Paddock, a reclusive 64-year-old high-stakes gambler, rained bullets on the crowd at a country music festival from his 32nd-floor hotel suite, killing 58 and wounding hundreds before taking his own life.

McMahill said investigators had reviewed voluminous video from the casino and don't think Paddock had an accomplice in the shooting, but they want to know if anyone knew about his plot beforehand.

In their effort to find any hint of his motive, investigators were looking into whether he was with a prostitute days before the shooting, were scrutinising cruises he took and were trying to make sense of a cryptic note with numbers jotted on it found in his hotel room, a federal official said.

The US official briefed by federal law enforcement officers wasn't authorised to discuss the matter publicly and spoke to the Associated Press on condition of anonymity.

The official said investigators were interviewing other call girls for information and looking into at least a dozen cruises Paddock took in the last few years, including one to the Middle East.

It is unusual to have so few clues five days after a mass shooting. McMahill noted that in past mass killings or terrorist attacks, killers left notes, social media postings and information on a computer, or even phoned police.

“The lack of a social media footprint is likely intentional,” said Erroll Southers, director of homegrown violent extremism studies at the University of Southern California. “We're so used to, in the first 24 to 48 hours, being able to review social media posts. If they don't leave us a note behind or a manifesto behind, and we're not seeing that, that's what's making this longer.”

What officers have found is that Paddock planned his attack meticulously.

He requested an upper-floor room overlooking the festival, stockpiled 23 guns, a dozen of them modified to fire continuously like an automatic weapon, and set up cameras inside and outside his room to watch for approaching officers.

In a possible sign he was contemplating massacres at other sites, he also booked rooms overlooking the Lollapalooza festival in Chicago in August and the Life Is Beautiful show near the Vegas Strip in late September, according to authorities reconstructing his movements leading up to the deadliest mass shooting in modern US history.

His arsenal also included tracer rounds that can improve a shooter's firing accuracy in the dark, a law enforcement official told the AP. It wasn't clear whether Paddock fired any of the illuminated bullets during the massacre.

Paddock bought 1,000 rounds of the .308-calibre and .223-calibre tracer ammunition from a private buyer he met at a Phoenix gun show, a law enforcement official not authorised to comment on the investigation said on condition of anonymity.

Tracer rounds illuminate their path so a gunman can home in on targets at night. But they can also give away the shooter's position.

Video shot of the pandemonium that erupted when Paddock started strafing the festival showed a muzzle flash from his room at the Mandalay Bay resort, but bullets weren't visible in the night sky.

McMahill said investigators are looking into Paddock's mental health and any medications he was on.

His girlfriend, Marilou Danley, told FBI agents on Wednesday that she had not noticed any changes in his mental state or seen indications he could become violent, according to a federal official who wasn't authorised to discuss the matter publicly and spoke to the AP on condition of anonymity.

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

Danley said she was unaware of any plans Paddock had when he sent her overseas to see family in her native Philippines. She was out of the country at the time of the attacks and has been labelled a “person of interest,” though she's not in custody and is cooperating with authorities.

Because so few people knew Paddock well, investigators will have a harder time probing his background for clues or hints he may have dropped about his plans, Southers said.There's “no one to say who's he mad at, what his motive is,” Southers said. “The key to this case right now is the girlfriend.”