The trigger for the riots in Urumqi appears to have been a recent deadly brawl in Guangdong between Uygurs and Han Chinese working at a Hong Kong-owned factory.
But the unprecedented eruption of violence - the worst on the mainland - in decades, in which at least 156 people were killed - underscored the simmering ethnic tensions in the mainly Muslim region.
This year is the 60th anniversary of the founding of the People's Republic of China and communist power.
The authorities in Xinjiang and overseas Uygur groups have blamed each other for the violence, which broke out on Sunday, but they seem to agree that the massive fight at a toy factory late last month in Shaoguan, northern Guangdong, lay behind the explosion of rage.
Authorities in Shaoguan said two Uygurs had been killed and 118 people injured in the brawl at the factory, owned by Early Light International, on June 25. They sought to play down the long-standing animosity between Uygurs and Han Chinese, which in recent years has turned increasingly violent in Xinjiang.
Xinjiang authorities said exiled Uygur groups seeking independence had used the Shaoguan incident as an excuse for Sunday's unrest.
Sources in Shaoguan said the authorities there had covered up the fact that a number of Han Chinese had been killed in last month's brawl. The clash was triggered by a rumour that six Uygurs had raped two young women working at the factory.