HSBC allowed fraudsters to transfer millions of dollars around the world even after it had learned of their scam, leaked secret files show.
Britain's biggest bank moved the money through its US business to HSBC accounts in Hong Kong in 2013 and 2014.
Its role in the $80m (£62m) fraud is detailed in a leak of documents - banks' "suspicious activity reports" - that have been called the FinCEN Files.
HSBC says it has always met its legal duties on reporting such activity.
The files show the investment scam, known as a Ponzi scheme, started soon after the bank was fined $1.9bn (£1.4bn) in the US over money laundering. It had promised to clamp down on these sorts of practices.
Lawyers for duped investors say the bank should have acted sooner to close the fraudsters' accounts.
The documents leak includes a series of other revelations - such as the suggestion one of the biggest banks in the US may have helped a notorious mobster to move more than $1bn.
