But mounting evidence suggests a system of forced labor is emerging from the camps, a development likely to intensify international condemnation of China’s drastic efforts to control and indoctrinate a Muslim ethnic minority population of more than 12 million in Xinjiang.

Accounts from the region, satellite images and previously unreported official documents indicate that growing numbers of detainees are being sent to new factories, built inside or near the camps, where inmates have little choice but to accept jobs and follow orders.

“These people who are detained provide free or low-cost forced labor for these factories,” said Mehmet Volkan Kasikci, a researcher in Turkey who has collected accounts of inmates in the factories by interviewing relatives who have left China. “Stories continue to come to me,” he said.