A gas station owner has been arrested for selling millions of liters of fake automotive fuel in South Korea, the Seoul Metropolitan Police Agency said Thursday.

Cho, identified only by his surname, produced and sold 12.3 million liters of fake gasoline and diesel in 11 gas stations in Seoul, Gyeonggi, and one altteul station – a thrift gas station – in Jecheon, North Chungcheong.

Two other men, Kim, 37, and Park, 59, who had helped Cho in producing the adulterated fuel were also arrested by authorities, Korea JoongAng Daily reports.

The scheme profited about 21 billion won ($19.4 million), police said.

The three men mixed gasoline and diesel with cheap kerosene and solvent. They added activated carbon such as charcoal to deflect tests for fake fuel, a technique one of the men learned from his uncle.

Authorities test for the chemical properties of kerosene to detect tainted fuel, but substances that determine whether the fuel is fake can be removed by adding activated carbon.

The fuel was transported in trucks and was parked in a hidden spot in the gas station.

“Fake fuel is one of the biggest elements that adds to the black market, so we will tighten control on fake fuel producing and selling with other agencies, such as the Korea Petroleum Quality and Distribution Authority,” said Shin Dong-shik, an investigator at the Seoul Metropolitan Police Agency.