BEIJING — Rukiya Maimaiti, a local propaganda official in China’s far west, warned her colleagues to steel themselves for a wrenching task: detaining large numbers of ethnic Uighurs and other Muslim minorities.

The Chinese government wanted to purge the Xinjiang region of “extremist” ideas, she told her co-workers, and secular Uighurs like themselves had to support the campaign for the good of their people.

“Fully understand that this task is in order to save your relatives and your families,” wrote Ms. Maimaiti, a Communist Party functionary who works on the western edge of Xinjiang, in a message that was preserved online. “This is a special kind of education for a special time.”

Her warning is one piece of a trail of evidence, often found on obscure government websites, that unmasks the origin of China’s most sweeping internment drive since the Mao era — and establishes how President Xi Jinping and other senior leaders played a decisive role in its rapid expansion.

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