Re: @W.S.Gosset - The Reg might note however
IIRC, HSBC was the initial whistle-blower.
But yes, the banks.
Basically, they were following up some oddities and uncovered the fake and falsely interstitial shell companies, and determined that the transactions were not just being hidden but had no other possible outcome or purpose than to weasel past US sanctions, but were still being piped through channels subject to US law. (Dumb as.)
To be fair, there was no patriotism or higher virtue in the banks' decision to flag it up, it was pure self-interest/self-protection. The banks at that point were themselves looking at major fines if it came to light later and it was clear they'd known. And the structure was so primitive and shitly done that it was like the Chinese had just read about the technique and had copied it but hadn't really understood it. So it was just a matter of time before it came to light.
If you haven't been paying attention, might be a good idea not to issue judgements about it or to attempt extremely self-embarrassing snark.