Austrian police says they had uncovered an online financial trading scam which according to initial estimates defrauded victims across Europe out of some €100m.
The operation allegedly used fraudulent online platforms purporting to trade in foreign and crypto-currencies, as well as high-risk instruments known as binary options.
There are thought to be several thousand victims across Europe, Vincenz Kriegs-Au of Austria's Federal Criminal Police Office told AFP.
The operation, which has been under investigation since mid-2017, is alleged to have used mass emails, social media and even its own call centres to reach victims.
While Austrian authorities were the first to uncover the websites, much of the infrastructure used by the network is in Bulgaria and the Czech Republic.
An international "joint investigation team" has been coordinating the probe with police in those countries.
In late January and early February there were raids on several properties in Bulgaria, resulting in the arrest of one "main suspect", along with the seizure of a six-figure sum of money and several terabytes of data, according to investigators.