A gruesome terror attack Thursday morning led to at least 31 deaths in Urumqi, capital of the far western Chinese region of Xinjiang. The attack — in which assailants in two cars plowed over shoppers and set off explosives in a crowded market area — is the worst such incident in years, surpassing a horrific slaughter in March, when knife-wielding attackers hacked down 29 people at a train station in the southwestern city of Kunming.

As in Kunming, authorities suspect ethnic Uighur extremists. My colleague William Wan explained in March who the Uighurs, a Turkic Muslim group, are and why Xinjiang, the region that comprises their homeland, is so restive. After the latest round of violence, it's worth unpacking further. Here are four underlying reasons why China's crisis in Xinjiang won't go away.

Ethnic politics

The Uighurs of Xinjiang are a distinct minority in China, a Silk Road people with a long, separate history centered around the oases and mud-brick towns of the Tarim Basin and the caravan routes toward Central Asia. The region's most storied city, Kashgar, was the birthplace, some argue, of the Turkish language — or at least of its first chronicler. For two spells in the 1930s and 1940s, Uighurs in Xinjiang declared independence under the banner of East Turkestan — a name and flag dissidents in exile still use to this day.

No matter the lip service to multiculturalism paid by Beijing, the Uighurs, not unlike the Tibetans next door, struggle with the hardship of being a minority group in an intensely centralized and authoritarian state. Uighurs face discrimination in major Chinese cities and, at home, look on as an influx of Han Chinese migrants radically reshape their homeland. Between 1949 and 2008, the proportion of the population of Xinjiang that is ethnic Han Chinese went from 6 to 40 percent. In Urumqi, Han Chinese now make up some three-quarters of the population. Ethnic riots there in 2o09 led to nearly 200 deaths.

Militant Islam

China is fighting its own war on terror, taking aim at Islamist separatists it views as part of a wider regional plague of extremism. The East Turkestan Islamic Movement (ETIM) is the group whose name is bandied about the most — though it's sometimes referred to as the Turkestan Islamic Party. ETIM is thought to have links with terror groups elsewhere, particularly in Uzbekistan, Afghanistan and neighboring Pakistan. Chinese authorities say ETIM has ties to al-Qaeda and training camps in the tribal area along the Pakistani-Afghan border. Uighurs were among the hundreds of supposed foreign fighters swept up and detained by the U.S. at Guantanamo Bay following the 2001 U.S. invasion of Afghanistan.

But all this doesn't amount to a direct causal link between organized transnational terror networks and the current epidemic of violence in Xinjiang. China blamed a bombing at a train station a month ago that killed three people on ETIM, but has now been more circumspect in pointing the finger at specific groups. It's unclear what kind of real operational capacity ETIM and other like-minded outfits have inside China and to what degree attacks like Thursday's are far more local actions.

Repression

Heavy-handed Chinese policies to combat militancy haven't helped the situation. Beijing warns against separatism and "splittism" and has arrested prominent Uighurs whose public statements and activities would in more democratic societies not be considered threatening. Human Rights Watch has documented a regime of mass detentions and enforced disappearances carried out by the security apparatus in Xinjiang.

China has curtailed Islamic religious practices in the region, razed much of the historic Old Town of Kashgar — a monument to Uighur culture leveled for the sake of "modernization" — and clamped down on other freedoms. After the 2009 violence, the government turned off the Internet in Xinjiang to curb the spread of messages that inflamed the riots, an act which radically altered the tapestry of life in Xinjiang for almost a whole year. Under former president Hu Jintao, the region's then-party secretary Wang Lequan downgraded the status of the Uighur language, deeming it unfit "for the 21st century" and making it an optional foreign language taught in state-run schools, akin to English.

Marginalization

Chinese government white papers point to the considerable economic and infrastructure developments brought about by Beijing in the region — which is rich in resources and sits at a strategic crossroads between China and Central Asia. Clearly, that's a narrative not accepted by all Uighurs.

While their disaffection remains palpable — and many observers call for a liberalization of Chinese policy in the region rather than an inevitable, harsh crackdown — the Uighurs also have very little leverage. While well-known, their plight is hardly a cause celebre internationally, and few regional governments would risk trade ties and other diplomatic links with China over the rights of one marginalized minority. In the past, Turkish Prime Minister Recep Tayyip Erdogan has expressed solidarity with the Turkic Uighurs, but even he sees little point in compromising relations with the world's next superpower, and promised China's President Xi Jinping on a visit in 2012 that Turkey would not allow any Uighur separatist activity to take place on its soil.

In an environment in which Uighurs have few means to agitate for change, it's perhaps not surprising that some choose such brutal, terrible methods to attract attention.