Las Vegas shooter Stephen Paddock likely had a severe mental illness that was probably undiagnosed, according to sources close to the police investigation.
FBI profilers have interviewed hundreds of people in the past week and in building up a picture of the high-stakes gambler found he had problems connecting with people, a report said.
They also believe he was aloof and found it hard to establish and hold down meaningful relationships, it added.
A search of Paddock’s room on the 32nd floor of the Mandalay Bay hotel turned up Valium tablets. The drug is used to treat a number of mental illnesses, from anxiety to panic attacks.
Police are struggling to find a motive for the attack, which left 58 dead and hundreds more injured, when Paddock opened fire from his hotel room onto concert-goers below.
Investigators have trawled through his life from birth to death and also combed through Paddock’s finances to see if he carried out the attack for money.
Officers also explored whether he may have had links to any radical groups and was driven by some specific ideology when he opened fire on the country music festival.
But they have failed to find a concrete reason for the attack despite chasing up 1,000 leads, according to sources who spoke to ABC News.
Beyond the belief that Paddock likely suffered from a severe mental illness, police are yet to establish a clear motive.
Kevin McMahill, of the Las Vegas Metro Police Department, told ABC News: “We thought we might find some ideology, some economic or political or social reason. Some medical reason. But we haven’t found it yet.”
Asked by a reporter if Paddock may have launched one of the biggest mass shootings in US history simply because he could, Mr McMahill didn't rule out the idea.
He said: “That’s certainly a possibility but it’s one of those possibilities you really can’t wrap your mind around. I don’t know if I can accept that.”
It came as police launched a fresh appeal for witnesses to help their investigation.
Their campaign includes billboards which have been mounted across Las Vegas that read: “If you know something, say something.” A free phone number to contact the FBI is shown.
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