The fraud squad has opened an investigation into London Capital & Finance, months after the investment firm collapsed into administration.
The Serious Fraud Office (SFO) said it is working with the City watchdog to investigate individuals linked to LCF.
Earlier this month, four people were arrested in Kent and Sussex and have subsequently been released pending further investigation, the SFO added.
The operation was co-ordinated with the assistance of the National Crime Agency and regional police forces.
LCF went into administration at the end of January after having its operations frozen by the Financial Conduct Authority (FCA) at the end of 2018 amid accusations of mis-selling.
The company netted £236 million after advertising tax-free individual savings accounts (Isas).
However, it was in fact a high-risk bond scheme with a high interest rate of 8%.
LCF did not have approval to advertise the product as an Isa.
The company’s largely elderly customer base fear they may not be able to recoup the money they invested with LCF.
Bondholders may get just 20% of their money back, according to LCF’s administrators.
Last week, the Financial Services Compensation Scheme (FSCS) said the mini-bonds issued by LCF were not regulated and therefore not protected by the scheme and it is not accepting claims against the firm.
The SFO is encouraging members of the public who invested in the scheme over the period 2016 to 2018 to contact it.