Chinese authorities have arrested over 1000 people involved in a huge illegal online gambling network.

On Wednesday, Guangdong province’s public security department announced the results of an operation dubbed ‘221’ that involved around 200 online gambling sites hosted on Thailand-based servers. Around a dozen individuals are believed to have set up the password-protected websites, which were then rented out to individual agents for between RMB 70k to 100k (US $11,300 to $16,100) per month.

Police began arresting some of these agents last June, just as the 2014 FIFA World Cup was getting underway. By December, a total of 1,071 individuals had been detained, and over 500 of these remain in custody. Bank accounts containing a total of RMB 330m ($53m) had been frozen but the authorities believe most of the betting money was transacted in cash and via illegal private banks.

The Xinhua news agency reported that these illegal sites collectively handled RMB 400b ($64.56b) in wagers per month from a total of over 400k gamblers. Bettors reportedly lost RMB 9b ($1.45b) to these sites in the month of December alone. Around 125 of the websites allowed bettors to wager on the results of the Shishicai national lottery, allowing the Ministry of Public Security to call the bust the largest lottery-related case on record.