An international drug-trafficking channel in southern China has been busted following a crackdown on a major cross-border drug gang in the Shenzhen special economic zone, police said on Friday.

In the crackdown, in the first half of this year, police seized more than 1.3 metric tons of cocaine in a raid on a drug distribution center in Shenzhen.

It was the largest drug case involving cocaine to have been cracked on the Chinese mainland, the public security bureau in Shenzhen’s Futian district said in a statement released on Friday.

The operation was launched by Shenzhen police in cooperation with counterparts from Guangzhou, capital of Guangdong province, and Zhuhai, it said. Police in the Hong Kong and Macao special administrative regions exchanged information and clues with the police from the Chinese mainland.

The cross-border drug-trafficking channel was mainly used to smuggle cocaine from South America to major ports on the east coast before moving it to Guangdong and neighboring Hong Kong and Macao by sea, the statement said.

Police seized 1,331 kilograms of cocaine valued at more than 1 billion yuan ($153.85 million) in the raid on the center run by the cross-border drug gang, it said, adding that it took more than 20 police officers three days to unpack the large quantity of cocaine.

Nineteen suspects, including several from Hong Kong with secret society backgrounds, were detained, and three vehicles used to transport the drug were also seized during the police operation, the statement said.

The special operation was launched after months of investigation, it said. Shenzhen police set up a special task force to focus on investigating and fighting the gang after they received reports from insiders in August last year.

The gang was controlled by drug traffickers who were Hong Kong residents, the statement said, adding that the crackdown had dealt a heavy blow to drug and related crimes in southern China.

A police officer from the Guangdong Provincial Department of Public Security said police across the province will not relax their vigilance in the fight against drug and related crimes in the years to come.

“Fighting drugs and related crimes is a tough and long-term task in Guangdong, which has been China’s main battlefield against drug crimes for several decades,” the officer, who requested anonymity, said on Friday.

Domestic and foreign drug traffickers have made persistent efforts to turn Guangdong, the window of China’s reform and opening-up, into a major drug distribution center, he said.

The officer promised an expansion of international cooperation in the fight against drug and related crimes. He also hinted that more special campaigns and operations will be launched to crack down on drug and related crimes across the province in the months to come.