Despite the government’s tightened grip on the rampant, unauthorized use of China UnionPay’s card system, authorities are reporting that the number of retail transactions using unregistered UnionPay point-of-sale terminals in the city state has more than quadrupled in 2016.

The city state’s Judiciary Police estimated that unauthorized China UnionPay transactions have reached MOP4.995 billion (USD626.5 million) last year, GGRAsia reported. That amount is about four times more than that MOP1.22 billion (USD153 million) calculated for the full year of 2015.

Macau police told the news outlet that they have conducted a total of 25 investigations involving unregistered UnionPay handheld terminals last year—20 of which were forwarded to the Public Prosecutions Office “for further action,” while the remaining five probes didn’t move forward as cases due to “lack of evidence.”

The police linked 53 people in the investigation: 14 from Macau, 38 from mainland China and one from Hong Kong, according to the report.