There have been more arrests in Urumqi, the capital of the Xinjiang Uyghur Autonomous Region, where for nearly two months the Chinese government has been trying to quell tensions between ethnic Uyghurs and Han Chinese.
There has been rising violence in recent weeks. During a protest on September 3, five people were reportedly killed. The following day, Han Chinese tried to break through police barricades to reach a Uyghur neighborhood.
China's central authorities have responded to the crisis by firing Li Zhi, the head of the Urumqi Communist Party, and the chief of police.
On September 6, police rushed to a protest outside the World Trade Wholesale Center Building in Urumqi after word spread that some Uyghurs had carried out a syringe attack.
The Chinese government says 531 people have filed reports of syringe attacks between July and September 4.
A Han Chinese man who said he saw the attack said he was confident that the attackers were Uyghurs. "There were two or three people arrested," he said. "They were Uyghurs."
A large crowd of Han Chinese assembled outside the building. Police responded by firing tear gas.
"There were many people gathered outside just now and the police fired five rounds [of tear gas]," one Han woman said. "Then many Han Chinese people were injured. A man beside me was bleeding from his ear. Even now my throat is still sore and my eyes were tearing for a while."
Getting accurate information out of Xinjiang is proving difficult. While Han Chinese are frequently shown speaking about their complaints and fears, Chinese media are not allowing representatives of the Uyghur community to present their side of events.
And Chinese authorities appear to be preventing foreign media from access to Xinjiang's Uyghur communities.
Unconfirmed reports of Han Chinese factory and plant owners abusing and killing Uyghurs sparked widespread riots on July 5.
There are approximately 8 million Uyghurs in Xinjiang. The Uyghurs are a Turkic Muslim people who have inhabited the Xinjiang region for many centuries.
Chinese geologists have found abundant oil reserves in Xinjiang's Tarim Basin, which has led to an influx of millions of Han Chinese workers.
Ethnic riots in July left nearly 200 people dead, most of them Han Chinese. In 1996, riots between Uyghurs and Han Chinese in another Xinjiang city, Yining, also left some 200 dead.
At times Uyghur rulers were allies of Chinese emperors, as was true during the Tang Dynasty from 618-907. But for most of their history, the Uyghurs have resisted Chinese attempts to annex the region.
The reports of syringe attacks have caused widespread fear among the Han Chinese.
Military doctor Xie Jianwei told a televised press conference on September 5 that so far there is no evidence the syringes were tainted with acid or the HIV virus, as rumors around Urumqi had claimed:
"From the cases that we have observed, the patients are doing well," Xie said. "In some, we find that there is swelling or changes to the puncture points on their skin, but their health is in good order."
China's official Xinhua news agency has reported that anyone who stabs others with syringes containing poisonous or harmful substances would face severe penalties, including the death penalty.