BEIJING—An overseas group advocating on behalf of China's mostly Muslim Uighur ethnic minority cast doubt Wednesday on the Chinese government's assertion that a recent outburst of deadly violence in the far-western Xinjiang region was the result of a terrorist attack.

On the same day, prosecutors in the Xinjiang capital of Urumqi announced that dissident Uighur economist Ilham Tohti, taken from his Beijing home by police in January, was formally charged with taking part in separatist activities.

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