Who are they?
"one of the most dangerous financial criminal groups" and "native English-speaking".
Easy. It's one of the big UK banks. They're always mis-selling something, rigging markets, interest rates or forex, or breaching money laundering rules, so moving more blatantly into financial crime is not even a diversification, more exploring a core competence.
Then again, US banks qualify under those two criteria, and the only people less trustworthy* than British banks are American banks.
* In the sense that they SHOULD be trustworthy but aren't. Let's face it, you're going to expect Russian, Syrian, Iranian, North Korean banks etc to be up to bad stuff.