The brother of Las Vegas shooter Stephen Paddock said something must have driven his brother to the "pit of hell" before he carried out a massacre of 59 people.
Eric Paddock, 55, is trying to raise US$1 million for victims by offering an exclusive television interview about his brother, who remains a mystery to detectives investigating the case.
He said his older sibling was a "private guy", but remains uncertain about what caused him to stockpile guns and ammunition in the Mandalay Bay hotel before firing on crowds below.
"Something happened that drove him into the pit of hell," he said.
Eric said he hopes police find a "tumour in his head or something" when they do an autopsy in order to explain what happened.
"If they don't, we're all in trouble," he said. "I'm praying for at least some data points. Because otherwise, the bug in 'Men In Black' put on a Steve suit and went and did this. There's no other rationalisation."
The clearly shocked younger brother of America's deadliest killer in recent history said he is still struggling to come to terms with what made Stephen snap.
"He was a private person. There's a story about that he's, 'ohhhh, he kept his shades closed, and he didn't talk to me for the first three times he saw me walking in the neighbourhood.' Wow. "That makes him really weird, doesn't it? He was a private guy," he said about the fact there are few clues as to what made Stephen decide to carry out the attack."
"That's why you can't find out anything about him, that's why there's no pictures. Is he such a weirdo because he didn't have a Facebook page and posted 50,000 damn pictures of himself every day? Who's weird?"
It comes as more details have been revealed about the "highly intelligent" gambler who favoured video poker machines and risked up to US$10,000 a day.
Eric said he was a "big fish" who was well known to local hotels due to his top tier membership card and was fun to hang out with because he was a "rich guy" often flying places.
"If a lot of these hotels say they don't know Steve, they're lying," he said.
But despite the financial security gambling provided for Stephen and his family, Eric said he treated it like a job rather than a passion.
"The casino was a means to an end. The casino to him was like a job in Toyota in Japan, where you live in the Toyota apartments across the street, and then you go to the Toyota factory to work. That's what the casino was. It's a place where you lived and they were nice to you, and you could get it paid for by playing slots."
Police are still trying to understand the motives for the shooting.
On Wednesday, authorities revealed the weekend before the shooting, Paddock had rented a high-rise condo in a building that overlooked the Life is Beautiful alternative music festival featuring Chance the Rapper, Muse, Lorde and Blink-182.