Investors in a £27m sports betting investment scheme which has now collapsed fear all their money will be lost if the authorities fail to act.
The Winning Express, which was promoted by ex-Premiership footballers Steve Claridge and David Hirst, offered huge, risk-free returns.
BBC Moneybox found authorities were repeatedly warned about the scheme, in which more than 1,000 people invested.
There is no suggestion Mr Claridge or Mr Hirst were guilty of wrongdoing.
The Winning Express was allegedly run by Graham Bradbury, 69, who had previously been banned from being a company director for running a pyramid scheme.
He died in Spain in November, the Foreign Office confirmed.
Investors in The Winning Express were guaranteed 2% tax-free interest per month - or 27% annually - through placing "sporting arbitrage" bets.
The scheme began in 2011, taking out newspaper and radio adverts in Spain, as well as later being promoted in a YouTube video by both Mr Claridge and Mr Hirst.
At first, investors received payouts as promised, and others signed up through word of mouth.
Commissions were awarded for recruiting new investors - a feature described by pyramid scheme experts as a "red flag".
Investor John O'Brien, a retired stockbroker who lives in Spain, says his family invested £100,000.
He said: "I wasn't convinced at first. But once it was explained I opened a Betfair account.
"I put £50 in it, had a go at arbitrage, and was pleasantly surprised that you made money. That convinced me that this was something really good."
What is sporting arbitrage?
Under the system, bets are placed with different bookies on all possible outcomes of a particular event, resulting in a profit or break-even whatever the result.
The technique exploits the fact that bookies offer varying odds on an event.
While it is possible to make money from sporting arbitrage, professional gamblers say getting rich is difficult, if not impossible.
Bookies close the accounts of those repeatedly betting on 'arbs'.
Sports data analyst Toby Aldous said: "They have exactly what the players have, a feed which tells them all the arbs which have happened.
"They can see who's betting on arbs all day and close their accounts."
Mr Aldous said there was a glass ceiling on how much players can make from sporting arbitrage. Told the interest rate offered by Winning Express, he said: "I do not believe that."
A document seen by the BBC lists 1,068 The Winning Express clients, with some individuals investing six-figure sums. The listed clients invested just over £27m in total and another £2.1m in a fund purportedly to invest in thoroughbred horses.
But in 2015, payments became erratic. Then, in September 2016, The Winning Express ceased operating.
Mr O'Brien, who heads a consortium taking legal action over the scheme, said: "Had the authorities been more on the ball, most of the clients would never have invested.
"Lives would not have been ruined.
"If the police act fast there's a chance it [remaining money] can be traced and returned to the people to whom it rightly belongs."
Every investor contacted by the BBC named Mr Bradbury as the man behind The Winning Express.
He was made bankrupt in 2013 over a pyramid scheme called Cherries that left investors £500,000 out of pocket. The Insolvency Service forbade him from "promoting, forming or managing" a company without the court's permission for 12 years.
The Winning Express was run through a company called Milton Express, established in the Seychelles, where directors have anonymity.
It operated in the UK via a third party company with offices in Sheffield.
The BBC has seen emails sent by Mr Bradbury connecting him to The Winning Express and a screenshot of the Milton Express bank account at Mauritius Commercial Bank bearing his name.
Rachel Adamson, a partner at Stephenson's Solicitors and pyramid scheme expert, said there were numerous red flags.
These included high interest rates with zero risk, Mr Bradbury's previous history and the use of initial returns to generate "good references".
The Winning Express was reported to the Financial Conduct Authority in May 2014 and the Serious Fraud Office by January 2015.
It was reported to Action Fraud in July 2015 and was also on the radar of the Gambling Commission, yet a formal investigation was only launched by South Yorkshire Police this September.
South Yorkshire Police said this week the investigation was international and therefore not for the force to pursue.
"The force is exploring which agency is the most appropriate to carry out such a complex and prolonged investigation," a spokeswoman said.
It is not clear what if any agency is now investigating.
All the authorities involved defended their own roles in the case, while the family of Mr Bradbury have made no comment.
Both Mr Claridge and Mr Hirst said they were not involved in the running of the arbitrage operation. There is no suggestion either was aware of suspicions over the scheme.
Mr Claridge, who invested in the scheme himself, insisted he did due diligence on the company before promoting it.