Dr Ruja, as she styled herself, was offering a radical new alternative that warm June evening in 2016 – a perfectly safe cryptocurrency of her own devising called OneCoin. The charismatic 36-year-old told the crowd that it was not too late to join a financial revolution that would become bigger than Bitcoin within just a couple of years

With flames leaping from the stage, ear-splitting music pumping from loudspeakers and a screaming crowd, the sell-out event at London’s Wembley Arena seemed more like a rock concert than a financial sales conference.

But as Alicia Key’s Girl On Fire began to fade, an electrified silence swept across the auditorium and Dr Ruja Ignatova took to the stage dressed in a flowing silk ballgown, and with bright red lipstick and spectacular diamond drop earrings.

Then, with thousands of eyes locked on her, the glamorous German-Bulgarian began to spell out an extraordinary vision.

The future, she said, belonged to cryptocurrencies such as Bitcoin, which had made millions for some small investors and were free of rules, regulations and the costly fees of traditional banks.

True, Bitcoin was complex and cumbersome for ordinary people.

But Dr Ruja, as she styled herself, was offering a radical new alternative that warm June evening in 2016 – a perfectly safe cryptocurrency of her own devising called OneCoin.

The charismatic 36-year-old told the crowd that it was not too late to join a financial revolution that would become bigger than Bitcoin within just a couple of years.

She urged them to invest. And the audience lapped it up, as they did at similar events in Dubai, Macau and Singapore. OneCoin promoters were hard at work in seminars and computer chatrooms around the world, in fact, and the reward was spectacular.

By March 2017, more than £3.5 billion had been invested in the OneCoin system from 175 countries. It is thought around £100 million of that came from the UK.

Yet within just a few months, the mysterious Bulgarian would be a fugitive from the FBI, accused of conducting one of the biggest individual frauds in financial history.

Today, her victims number more than a million, many facing financial ruin. Dr Ruja, meanwhile, has vanished.

As the sheer scale of the fraud became apparent, I set out to find Dr Ruja, starting at her £2 million mansion in Bulgaria’s seaside resort of Sozopol. Her impressive yacht (above) was moored in a nearby harbour, but the staff said they hadn’t seen her since 2017

Just as promised, OneCoin was simple to understand. Investors could sign up to buy the currency which would then appear in their account.

Because more and more people would join, said Dr Ruja, the value of the currency would go up and, sure enough, early investors soon saw the value of their holdings rise.

One day very soon, these coins could be traded for real money, they were assured. And before long, shops and restaurants would accept OneCoin as payment too.

Supporters described Dr Ruja as the next Mark Zuckerberg. She was said to have an IQ of more than 200.

The Bulgarian was pictured on the cover of Forbes magazine, which specialises in wealthy individuals.

Social media was awash with pictures and clips of Dr Ruja, including a dazzling performance at a European conference organised by The Economist.

Blessed with a magnetic personality and good looks, she wielded a cult-like influence, according to disillusioned former followers.

Investors developed their own hand-sign – making an ‘O’ with the thumb and index finger – and referred to each other as ‘family’.

There was a life-size cardboard cut-out of Dr Ruja in the company’s office, which adoring staff used to touch as a ritual before making the OneCoin sign –until it eventually fell to pieces.

With interest rates at rock-bottom levels, the appeal of cryptocurrencies and the rapid gains they were making seemed obvious to ordinary savers. Where else could they put their money?

Early doubts were expressed about OneCoin in Finland, Sweden and Belgium, where the authorities issued investor warnings. Readers of specialist financial websites were told that OneCoin bore the hallmarks of a pyramid-selling scheme.

Social media was awash with pictures and clips of Dr Ruja, including a dazzling performance at a European conference organised by The Economist. Blessed with a magnetic personality and good looks, she wielded a cult-like influence, according to disillusioned former followers

But such criticism did nothing to stop the money pouring in. Dr Ruja’s supporters dismissed the sceptics as ‘haters’, either afraid of the revolution or unable to comprehend it.

After all, what was there to doubt? OneCoin was going from strength to strength. There was still no sign of being able to sell OneCoin for dollars or pounds, but money was being made.

In 2016, when Dr Ruja spoke in London, plenty of people were making sizeable cash commissions by selling OneCoin to friends and family.

And how could Dr Ruja be anything other than trustworthy? She obtained a law degree from Oxford, followed by a stint working for McKinsey & Company, the respected global asset management firm.

The Bulgarian herself was reassuringly rich, throwing expensive parties on her luxury yacht moored in the Black Sea. She based herself in a rented penthouse in London’s Kensington, where she celebrated her 36th birthday.

A video posted on YouTube shows guests enjoying pink champagne, vodka and sushi at the V&A museum, where hundreds of revellers, all in ballgowns or black tie, danced to a live performance by Tom Jones.

By the autumn of 2017, the self-styled ‘cryptoqueen’ seemed unstoppable. But in October, she failed to appear at a OneCoin event in Portugal.

Frantic phone calls, emails and messages went unanswered. No one had heard anything at OneCoin’s head office in the Bulgarian capital, Sofia.

And it was then, for the first time, that most of Dr Ruja’s millions of victims began to realise something was seriously wrong.

Dr Ruja’s scam, it turns out, was surprisingly simple. And as I started to investigate how ordinary savers were sucked in, I came across Jen McAdam, 48, from Glasgow, who had invested £900 in the cryptocurrency after seeing a presentation online.

Like many OneCoin investors, she didn’t really understand the technology but was blown away by the amazing returns on offer and, most of all, by the impressive leader.

She signed up and was sent an activation code, giving her access to a website where she could check the value of her OneCoins.

Very quickly, she could see her portfolio rise. As far as she was concerned, it seemed like easy money.

Impressed, she invested her £8,600 inheritance – her coal miner father’s life savings – into the scheme. And she encouraged friends and family to join.

Some people who have had their fingers burned by OneCoin are beginning to talk. Various sources say Dr Ruja has sought refuge in Dubai, Russia, Brazil and even London. There are rumours that she might be getting help from higher powers – organised crime, perhaps, or even the Kremlin

Fairly soon, £200,000 had been invested by her Glasgow-based network. Tearfully, she told me that ‘it spread like wildfire’.

After just a few months, Jen believed her original stake was worth nearly £100,000. But she would soon be disabused of that notion.

One evening, she received a disturbing call from Timothy Curry, a Bitcoin enthusiast from California, who was trying to warn investors that OneCoin was a scam and that their money was gone. She didn’t believe him at first.

But Curry explained the basic principle that all cryptocurrencies have a special type of database called a blockchain – an incorruptible computer record of all transactions that proves to everybody that the currency is sound.

This technology makes it impossible to counterfeit new currency and guarantees there is a finite number in circulation.

Without the security of the blockchain and the fiendish mathematics behind it, the price could be rigged or faked. Whole fortunes could be created and then deleted at the push of a button.

Jen started asking to see evidence of OneCoin’s blockchain, but the company’s officials refused, saying it was secret.

After speaking to a blockchain specialist, she came to the horrifying realisation that the daily valuation of her investment on her computer screen did not reflect any true value at all. It was just a made-up price created in Bulgaria.

OneCoin wasn’t part of the cryptocurrency revolution. It was an old-fashioned confidence trick dressed up in high-tech clothing.

The disaster has taken on global proportions. Because OneCoin was sold through network marketing, people like Jen brought in friends and family.

Farmers in Uganda invested a small fortune hoping it would transform their lives. A disproportionate number of British Muslims are among the victims.

Forbidden by sharia law from many standard forms of financial investments –because of the prohibition on earning interest – OneCoin seemed ideal and it seems they were targeted.

So how was Dr Ruja able to convince people across the globe to invest in a such a scam?

Eileen Barker, professor emeritus of sociology at the London School of Economics, believes the OneCoin organisation had similarities to messianic movements or cults in which people are led to believe they are part of something so big it is going to change the world.

The regulators, moreover, were feeble. The UK’s Financial Conduct Authority issued a warning about OneCoin in September 2016, but removed it in August 2017 with little explanation. The FCA now faces serious questions.

OneCoin promoters took this as evidence nothing was wrong and carried on recruiting.

Shockingly, people are still selling OneCoin in Britain today and still sharing that Forbes ‘cover’ picture on social media. (It was, in fact, nothing more than a paid advertisement in a Bulgarian edition of the magazine.)

Private emails released in a bundle of US court documents suggest Dr Ruja had an exit strategy planned soon after the company was set up: ‘Take the money and run, and blame someone else…’ she wrote.

But the documents suggest that at other moments she acknowledged the gravity of her actions, writing to one colleague: ‘The damage is done. I have to somehow live with it. But it is something that really upsets me.’ Not that this will be much comfort for the victims she has left behind.

In March this year, Dr Ruja’s younger brother Konstantin Ignatov, who took over OneCoin when Ruja vanished, was arrested by the FBI as he boarded a plane from Los Angeles to Turkey.

He was charged with fraud, and Dr Ruja was charged in absentia with wire fraud, securities fraud and conspiracy to commit money-laundering.

Today, Dr Ruja is wanted by the FBI. One of New York’s top prosecutors, the District Attorney for Manhattan, has described OneCoin as ‘a pyramid scheme based on smoke and mirrors’.

And like all pyramid schemes, it’s only the people at the top who get very rich.

While most cryptocurrencies, including Bitcoin, are perfectly legitimate enterprises, Dr Ruja found she could exploit the hype surrounding new technology, the lack of effective regulation – and plain, old-fashioned ignorance.

The FCA says British citizens have lost millions in crypto scams, mostly in speculative new projects they didn’t understand.

As the sheer scale of the fraud became apparent, I set out to find Dr Ruja, starting at her £2 million mansion in Bulgaria’s seaside resort of Sozopol. Her impressive yacht was moored in a nearby harbour, but the staff said they hadn’t seen her since 2017.

I found parts of her multi-million-pound property empire in Sofia: the city’s most famous restaurant – now boarded up – and an imposing building overlooking Bulgaria’s parliament. But of Dr Ruja herself, I found nothing.

And what about the missing fortune? The Mail on Sunday understands that a proportion of the £3.5 billion she accumulated is managed by a UK family wealth office in London, but so far our questions have gone unanswered.

Meanwhile, a US lawyer is currently on trial in New York accused of laundering OneCoin proceeds.

I was startled to find the OneCoin headquarters operating as normal in Sofia. OneCoin insists the US authorities are wrong, that it is a legitimate cryptocurrency and that everything will come good in the end. The company was unable to help me find its founder.

Some people who have had their fingers burned by OneCoin are beginning to talk.

Various sources say Dr Ruja has sought refuge in Dubai, Russia, Brazil and even London.

There are rumours that she might be getting help from higher powers – organised crime, perhaps, or even the Kremlin.

Some say she is living on a yacht and travelling around the Mediterranean with a fake passport and that her appearance has been altered with cosmetic surgery. Others are convinced she is dead.

Then, two weeks ago, I got another lead. Could she be in Germany, where Dr Ruja’s family emigrated when she was ten years old?

Hours of research on the internet, cross-referencing the movements of her friends and associates, suggested that the most likely hiding place is Frankfurt.

It might seem a gamble to hide from the FBI in Germany’s financial capital, but Dr Ruja is a woman used to taking risks.

And with access to so much money, she could have multiple identities, addresses and a regularly changing appearance – as disturbingly intangible as the so-called currency through which she has stolen billions from ordinary, hard-working people.

Jamie Bartlett’s eight-part podcast, The Missing Cryptoqueen, is available on BBC Sounds.