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Las Vegas mass killer Stephen Paddock may have had a severe undiagnosed mental illness, according to investigators.

FBI profilers and behavioral scientists have been trying to better understand what drove Paddock to open fire on the music festival crowd, leaving 58 people dead and injuring nearly 500.

According to ABC News, following interviews with hundreds of people over the past week, the picture being painted of Paddock is someone who had trouble interacting with people.

A source told the publication that the 64-year-old - who spent hours gambling in casinos with little human contact - has been described as standoff-ish, disconnected and someone who had trouble maintaining meaningful relationships.

They added that he was showing many antisocial traits that are typical of past mass shooters.

(Image: Getty)

Clark County Undersheriff Kevin McMahill said investigators remain largely in the dark about what drove retired real estate investor and high-stakes gambler Paddock to carry out the deadliest mass shooting in modern US history.

"We have looked at everything, literally, to include the suspect's personal life, any political affiliation, his social behaviors, economic situation, any potential radicalization," McMahill told reporters late on Friday.

"We have been down each and every single one of these paths, trying to determine why, to determine who else may have known of these plans."

(Image: Twitter)

McMahill acknowledged that Islamic State had repeatedly claimed responsibility for the attack, but said investigators had uncovered "no nexus" between the Mideast-based militant group and Paddock.

In an unusual bid to cast a wider net for tips, the FBI and police have arranged with communications company Clear Channel to post billboards around Las Vegas urging citizens to come forward with any information they believe might help investigators.

The billboards will bear the slogan, "If you know something, say something," and carry a toll-free number to an FBI hotline, said Aaron Rouse, special agent in charge of the Las Vegas FBI office.

Paddock, 64, unleashed a torrent of gunfire onto an outdoor music festival from the windows of his 32nd-floor hotel suite overlooking the concert on Sunday night, then shot himself to death before police stormed his room.

(Image: AFP)

In addition to the 58 people who died, nearly 500 were injured, some by gunfire, some trampled or otherwise hurt while running for cover.

Unlike so many other perpetrators of deadly mass shootings before him, Paddock left behind no suicide note, no manifesto, no recordings and no messages on social media pointing to his intent, according to police.

(Image: AFP)

McMahill said investigators remained certain Paddock acted alone in the shooting. But police have said they suspect he had help before the killings, based on the large number of guns, ammunition and explosives found in the hotel suite, his home, his car and a second home searched in Reno.

Authorities have said that 12 of the weapons recovered from Paddock's hotel suite were equipped with so-called bump-stock devices that enable semi-automatic rifles to be operated as if they were fully automatic machine-guns.

Paddock's ability to fire hundreds of rounds per minute over the course of his 10-minute shooting spree was a major factor in the high casualty count, police said.