Investigators believe Las Vegas gunman Stephen Paddock hired a prostitute in the days leading up to the shooting and were interviewing other sex-trade workers for information, a U.S. official briefed by federal law enforcement officials said.

The official wasn't authorized to discuss the matter publicly and spoke to The Associated Press on condition of anonymity.

It is unusual to have so few hints of a motive five days after a mass shooting. In previous 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.

An Australian friend of Las Vegas shooter Stephen Paddock says "we'll never know" why Paddock carried out the massacre.

The friend, businessman Adam Le Fevre, said about Paddock, "Everything he did seemed to be planned with precision."

Le Fevre used to be in a relationship with the sister of Paddock's partner, Marilou Danley. The couples made two trips together to the Philippines, and Le Fevre and Liza Werner visited Danley and Paddock in Las Vegas in 2015.

Paddock was a strong gun rights advocate, said Le Fevre, who made no mention of his other political views in a television interview broadcast on the program A Current Affair on Australia's Nine Network Friday night.

'Not a loving relationship'

Le Fevre said it was "not a loving, caring relationship" between Paddock and Danley. "He would talk to her in a condescending way, and while I was concerned, I thought it was part of his nature."

Danley wouldn't have known about Paddock's plans, Le Fevre said. "I believe if she had the slightest inkling something was not right, she would have shared that with her family and would have potentially been able to make a change."

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Workers on Friday board up a broken window at the Mandalay Bay hotel, where Stephen Paddock fired from during his mass shooting five days earlier. (Chris Wattie/Reuters)

Le Fevre said he and Werner were accommodated in "out of this world, penthouse suites like you wouldn't believe," thanks to Paddock's "high-roller" status with Vegas casinos.

He said that Paddock told him, "As a professional gambler I operate with $1.5 million to $2 million a year and that can generate me anywhere between $200,000 to $300,000 a year."

Le Fevre also said that Paddock talked about sex-trade workers the casinos offered him, and that he believed some of those offers had been accepted.

Arsenal included tracer bullets

Paddock's arsenal included tracer rounds that can improve a shooter's firing accuracy in the dark, a law enforcement official told The Associated Press on Friday.

It wasn't clear whether Paddock fired any of the illuminated bullets during the highrise massacre Sunday night, but his purchase of 1,000 rounds provided more evidence of how methodically he planned the attack that killed 58 people and injured nearly 500.

Four Canadians are among the dead.

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

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

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A bump stock device, left, fits on a semi-automatic rifle to increase the firing speed, making it similar to a fully automatic rifle. (George Frey/Getty Images)

Video shot of the pandemonium that erupted when Paddock started strafing the country music festival showed a muzzle flash from his 32nd-floor room at the Mandalay Bay resort, but bullets weren't visible in the night sky.

Paddock's preparations included requesting an upper-floor room overlooking the festival, stockpiling 23 guns. A dozen of them included bump stocks, attachments that can effectively convert semi-automatic rifles into fully automated weapons. He also set up cameras inside and outside his room to watch for approaching officers.

The 64-year-old high-stakes gambler killed himself as police closed in on his room.

In a possible sign he had contemplated 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 recent U.S. history.

A federal official said authorities are looking into the possibility Paddock planned additional attacks, including a car bombing. The official wasn't authorized to discuss the matter publicly and spoke to The Associated Press on condition of anonymity.

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Fans gather in Chicago's Lollapalooza musical festival in Grant Park in August. Las Vegas gunman Stephen Paddock had booked a room overlooking the festival. (Rob Grabowski/Invision/Associated Press)

Paddock had 1,600 rounds of ammunition in his car, along with fertilizer that can be used to make explosives and 50 pounds of Tannerite, a substance used in explosive rifle targets.

Marilou Danley told FBI agents Wednesday that she had not noticed any changes in his mental state or indications he could become violent, the federal official said. Paddock sent Danley on a trip to her native Philippines before the attack, and she was unaware of his plans and devastated when she learned of the carnage while overseas, she said in a statement.

Memorials begin

On Thursday night, thousands raised candles and surrounded the widow and two children of officer Charleston Hartfield, who was killed in the shooting.

Hartfield, a 16-year army veteran who served in Iraq, was known as Charles, Chuck and ChuckyHart, but one longtime friend and colleague dubbed him Captain America.

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Veronica Hartfield, left, widow of slain Las Vegas police officer Charleston Hartfield, and their son, Ayzayah Hartfield, 15, attend a vigil in Las Vegas. Charleston Hartfield, who was off duty at the Route 91 Harvest country music festival, was killed in the massacre. (Robyn Beck/AFP/Getty Images)

"Charlie Hartfield was the greatest American I have ever known," Sgt. Ryan Fryman told the crowd.

His was among the first memorials for the dead, whose identities have now all been released by authorities.

The victims killed ranged in ages from 20 to 67. Two of them, 24-year-old Austin Cooper Meyer and 61-year-old Brett Schwanbeck, had not been identified before the Clark County coroner released a complete list Thursday night.

Police announced Thursday they had found a Hyundai Tucson SUV they had been searching for as part of the probe while executing a search warrant at the home in Reno that Paddock shared with Danley. It wasn't immediately clear if the vehicle was found on Thursday or earlier in the week when police searched the home and found several guns and ammunition.

Investigators combing through his background for clues remain stumped as to his motive.

The profile developed so far is of a "disturbed and dangerous" man who acquired an arsenal over decades, Sheriff Joseph Lombardo said. But investigators have been frustrated to find that he lived a "secret life," Lombardo said, "much of which will never be fully understood."

A former executive casino host at the Atlantis Casino Resort and Spa in Reno said Paddock had a "god complex" and expected quick service without regard to how busy the staff was at the time.

"He liked everybody to think that he was the guy," John Weinreich said. "He didn't boast about anything he had or anything. It was just his demeanour. It was like, 'I'm here. Don't cross me. Don't look at me too long.'

With files from CBC News