求翻译：Two years ago in Xinjiang's regional capital of Urumqi, simmering tensions erupted in the region's worst ethnic violence in more than a decade. Uighurs attacked Han Chinese, overturning buses and cars and torching shops in a riot the government says killed 197 people, mostly Hans. In the aftermath, hundreds were arrested in a region-wide crackdown and about two dozen sentenced to death. Many other Uighurs remain unaccounted for and are believed to be in custody.是什么意思？

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