Visitors to Matthew Mellon's James Bond-style eyrie high above Los Angeles's famous Sunset Strip soon became exposed to the unconventional behaviour of the vastly wealthy banking heir.

As soon as they were ushered through the 30ft white marble and stainless steel front doors, an aide would hand them a special plastic sticker to put over the camera lens of their phone, so 'terrified' was Mellon of intrusion.

Gun-toting bodyguards patrolled the £100,000-a-month ($194,200) rented property, which had CCTV cameras on every ceiling and corner – even in the private cinema with its red leather seats, and the downstairs spa complete with massage tables, steam room, sauna and outdoor hot-tub.

"Matthew was paranoid about security," says his personal assistant and "sober coach" Jesse Bias, 28. "He always thought someone was out to steal his millions. He trusted no one."

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Mellon, 54, died in Cancun, Mexico, last Monday. He had been due to check into a rehab facility in a desperate attempt to quit his drug habit, which included smoking heroin and crack cocaine, and taking, by his own admission, up to 80 powerful prescription opioid painkillers every day.

While his family – he was part of the Mellon banking dynasty and his British first wife Tamara Mellon founded the Jimmy Choo shoe empire – said he died of a heart attack, a Mail on Sunday investigation has revealed that those closest to him during his last turbulent months fear something more sinister may have befallen the handsome father of three.

"People are whispering about whether he was murdered for his cryptocurrency," one of his close friends said last night.

"Hundreds of millions of dollars are missing, possibly for ever. Matthew always said 'they' were out to get him. Perhaps he was right?"

Matthew Mellon's family have said he died from a heart attack but those closest to him fear something more sinister may have befallen him. Photo / Getty Images
Matthew Mellon's family have said he died from a heart attack but those closest to him fear something more sinister may have befallen him. Photo / Getty Images

At the time of his death, Mellon had been due to check into the Clear Sky Recovery centre in Cancun for controversial ibogaine treatment, which uses a hallucinogenic drug to quell cravings. The treatment is banned in the UK and US.

As well as his well-publicised battle with addiction, he was bipolar – a condition he inherited from his father. He also inherited his family's extraordinary business acumen.

According to Forbes magazine, Mellon was worth an estimated $1 billion ($1.38b) because of an extraordinary gamble in Ripple cryptocurrency –money similar to Bitcoin which exists only in the digital world. He turned an initial $2 million investment into an online fortune.

He "cashed out" $350 million of his investment on January 3 – just six days before the value of Ripple plunged by 80 per cent. The market has since rallied, meaning his remaining investment is currently worth about a quarter of a billion dollars – and rising. But a disturbing question remains about where the money is.

Mellon told friends his fortune was hidden in dozens of secret accounts only he could identify and that he had hidden the access codes in bank vaults all over America. The only problem is that now he is dead, who can tell whether any of it is missing?

Jesse, his closest aide who was with him "24/7" for the last six months of his life, said his boss believed his colossal windfall in the volatile cryptocurrency market made him an obvious target. He said: "He was very kind and generous and people took advantage. He became paranoid, worried that people were out to harm him and take his money."

Lady Victoria Hervey, a close friend who was encouraged to study the nascent cryptocurrency business by Mellon, added: "When Matthew started going on about Ripple, everyone thought he was nuts. His family didn't believe in him – no one did. Everyone thought it was just another of Matthew's harebrained schemes.

"He took a gamble with $2 million from his trust fund and it paid off. But Matthew was so paranoid he hid his cryptocurrency under false names. He was terrified of hackers.

"He told me the codes were in 'cold storage' on nano-memory cards in banks dotted around America.

"The question is, will anyone ever be able to find it?"

The extraordinary saga of how Mellon lost and then made an incredible fortune is so bizarre, some of his Hollywood friends are already talking about turning his life story into a movie.

Born into incredible wealth and privilege, he was tormented by inner demons following the tragic suicide of his father Karl in 1983 when Mellon was just 19. He reportedly received $25 million when he turned 21 but most of his fortune was locked in a trust and he constantly complained of being broke.

First wife Tamara, mother of his 16-year-old daughter Minty, told how he would "disappear" for days on cocaine binges, something that led to the eventual collapse of their marriage.

A second marriage to New York designer Nicole Hanley in 2010 produced two more children but ended in divorce in 2015. Mellon told friends he was "heartbroken" that his drug problems meant he did not see as much of his younger children as he wished.

He moved to Los Angeles two years ago in a bid to repair his relationship with eldest daughter Minty. "He desperately wanted to be clean and sober for his children's sake and his own," said Lady Victoria, daughter of the 6th Marquess of Bristol.

She spoke to Mellon shortly before he left on his ill-fated trip by private jet to Cancun. "When you are as rich and generous as Matthew, there are always people ready to take advantage," she said. "There were people around him who encouraged his addictions and led him down some dark paths."

Despite his trust fund, Mellon found he did not have the access to sufficient funds to live the life he aspired to. Jesse, who lived with Mellon, said: "Before the cryptocurrency windfall, he was cash-poor. Last year, he was down to his last $500. I was with Matthew one day and he wanted to buy a Ferrari for half a million dollars. He called his trust officer who said, 'You don't have the money.'"

But Mellon developed an early interest in Bitcoin and other virtual currencies, and formed the company MellonDrexel with partner David Marshack, who said: "Matthew was very interested in the idea of what Bitcoin could be very early on.

"He didn't understand the underlying 0s and 1s of the digital technology but he understood that technology could make things cheaper, faster, more efficient.

"He risked everything on it and towards the end of last year, the market exploded and made him an awful lot of money."

But with the money came added temptation. After yet another stint in rehab last year – Mellon bragged to friends he had been to 50 clinics – the businessman slipped into a "terrible" addiction to heavy drugs by the end of the summer of 2017, which only worsened by the start of this year.

"He couldn't be left alone for a second," said Jesse. "I was there when he overdosed and I had to call 911 and rush him to hospital." On another occasion he fell down stairs while carrying a glass bowl and nearly severed a finger.

Mellon had been to the Cancun clinic for two weeks in January after friends staged an intervention, which included bringing Minty to his house. "It took us eight hours to persuade him to get on the plane," said Jesse. "But eventually he went."

Despite his troubles, Mellon had an incredibly kind and generous side. His friends describe him as a brilliant raconteur who charmed the ladies and was always impeccably dressed.

Jesse said: "We would go to 7/11 [a corner store] and he'd come out and see a homeless person and give him $1000 cash. When the mother of a friend of mine had cancer, he immediately said, 'I have to pay for her treatment.'"

"He had never earned his own money before," said Lady Victoria. "When he finally did, he was determined to enjoy it – like a giant f*** you to the world."

Mellon rented one of the most expensive homes in LA, perched on a hillside above the famed Chateau Marmont hotel where Blues Brothers actor John Belushi died of a heroin and cocaine overdose.

He set about enjoying his wealth amid full-sized Buddha statues and a large artwork in the master bedroom which read "Golddigger".

"We were supposed to meet for lunch one day but instead he called and told me to meet him at the Ferrari dealership," recalled Lady Victoria. "He bought three Ferraris on the spot."

Shortly before his death, he laid out $1 million to rent a Malibu beach home for the summer. He was having a new private jet made, and he planned to rent a mega-yacht for the Monaco Grand Prix.

Yet he was a troubled and conflicted soul who would attend meetings of a 12-step programme every day at 7.30am. He would down smoothies and wheatgrass, have vitamins infused into his body via an intravenous drip, and "freeze" himself in cryotherapy sessions. He even told Lady Victoria he planned to have himself cloned. He instructed his private chef to cook only organic food but would spend every night partying.

Lady Victoria said: "Being with him was like being on a wild rollercoaster ride. He was like a real-life James Bond. He'd get bored of his Ferrari so would drive to a Lamborghini dealer and buy a new car. But his money attracted bad people."

Despite insisting to friends in London he was "clean and sober" just a few weeks ago, his drug use – and his paranoia – were spiralling out of control. He had four mobile phones, including one he called "the Batphone".

Last weekend, he flew in a private jet to see his mother and stepfather in Florida before flying on to Cancun, where he checked into a suite at the five-star Villa del Palmar beach resort.

A hotel source said: "Mr Mellon arrived with two bodyguards and booked into a suite for seven nights. He collapsed within hours of checking in late on Sunday night and was rushed seven miles by ambulance to the Hospiten hospital in Cancun, where he died."

Mellon was taken from the hospital for a post-mortem examination, according to a funeral home proprietor in the city.

Fernando Diaz, of Jardines de Paz funeral service, said he understood from his contacts that Mellon "died of unnatural causes". After the post-mortem, Mellon's body was taken to Breton funeral home, where he was cremated and his ashes sent to his family in America.

Many friends and family are due to gather for his funeral service at St Joseph's Episcopal Church in Boynton Beach, Florida, on Friday. It will be a time for reminiscing and for grieving.

But many will also be silently wondering about the fate of his missing millions.

Additional reporting: Greg Woodfield in Cancun