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Why did the Shanghai checkpoint arrest people?

Illegal transport of dangerous substances. Shanghai Border Inspection Station is mainly engaged in entry-exit frontier inspection, ship driving and maintenance. The reason for detaining people is illegal transportation of dangerous substances. People's Republic of China (PRC)-Shanghai Entry-Exit Frontier Inspection Station, referred to as "Shanghai Frontier Inspection Station" for short, is under the State Administration of Immigration and is staffed by the people's police.