Macau police have rumbled a gang that modified mobile banking terminals to appear as if users were still on the Chinese mainland.

On Wednesday, Macau’s Judiciary Police (PJ) announced the arrests of 21 individuals suspected of illegally altering point-of-sale (POS) mobile terminals to avoid China’s strict rules about taking money out of the country. Mainland police arrested an additional 16 suspects in simultaneous raids across several provinces.

The modified POS machines allowed Macau visitors to use their bank cards to access cash while making it appear as if they’d never left the Chinese mainland. The PJ said the gang had reportedly begun its illegal operations in 2016 and allowed users to illegally withdraw MOP1.4b (US$173.4m) in 2018 alone. The gang’s profits reportedly funded a much larger underground banking network.

The PJ’s investigation began in November 2018 after receiving certain information from their mainland counterparts. The PJ raided eight luxury flats in Macau, seizing a number of the modified POS terminals and around HK$5m in cash.