Processing online gambling transactions in China could get a lot tougher after authorities broke up a major underground banking operation.

On Thursday, the Ministry of Public Security announced that police in Zhejiang province had detained around 100 suspects connected with an illegal banking system that handled transactions totaling RMB 410b (US $64.2b). The authorities are calling it the largest ‘grey capital’ takedown in the country’s history.

The suspects reportedly belonged to a network of eight gangs that processed transactions via dozens of Hong Kong-based shell companies on behalf of as many as 14,000 clients. The ring stands accused of helping Chinese residents launder money earned via illicit means, including online gambling sites dealing with Chinese bettors.

Chinese authorities began investigating the ring in September 2014 and warrants were issued three months later. It has reportedly taken this long to sort through the complex web of affiliated companies that handled 1.3m individual transactions via 850 different bank accounts.