“They have all gone through the training, political review and physical examination,” Cheng Peng, a local official who accompanied the workers, said in a telephone interview from the train. “The training subjects include ethnic unity, the law and Mandarin Chinese, as well as the factories’ regulations and routines.” Ethnic unity training “is about how the ethnic minorities communicate with mainland Han people, like in etiquettes and manners,” he added.

Because of recent outbreaks of violence, including a deadly knife attack carried out by Uighurs last March in a train station in southwest China, “people have a bias against Xinjiang people,” Mr. Cheng said. “We need to establish a new image.”

Mr. Cheng is an official in Shufu County near Kashgar, which he said had been sending laborers across China since 2006. Though neither he nor the state media reports mentioned it, the county’s policy of exporting Uighur labor set the stage for a factory brawl that led to deadly rioting in Urumqi in 2009, in which at least 200 people were killed.

The brawl took place at a toy factory in Shaoguan, Guangdong, in June 2009, after several weeks in which 800 Uighur workers from Shufu County had gradually arrived at the factory. Rumors that Uighurs had raped two Han women spread among the factory’s 16,000 Han workers, and a four-hour melee erupted. Two Uighur men were killed and 120 other people were injured, according to official reports.

A large protest by Uighurs in Urumqi followed, calling for an investigation of the Guangdong violence. That culminated in the Urumqi riot, which was the deadliest outburst of ethnic violence in China in decades.

At that time, people in the Kashgar area said various means had been used to ensure the export of Uighur labor, including threatening reluctant families with enormous fines. Some Uighur men said they were particularly upset at the recruitment of young, unmarried Uighur women. Some families, though, said they welcomed the economic opportunities provided by the labor export program.