The last time I remember not being able to pick up bread from a corner store in Karachi was in December 2007 in the days following Benazir Bhutto's assassination. But last week Pakistan's city that never sleeps again fell utterly silent as its residents, scared of rioting, firing, and arson, locked themselves indoors for three days - many giving up daily wages -while over 85 people were killed and billions of rupees in earnings were lost in the country's commercial capital. Since then, at least 12 more people have died in continued targeted killings. The empty roads, closed gas stations and shuttered shops were brought on by the murder last Monday of provincial assembly member Raza Haider, a leader of Karachi's dominant political party, the Muttahida Qaumi Movement (MQM). But while they were eerily reminiscent of the tense hush two and a half years ago, this time they have a far less simple explanation.

The defining characteristic of the history of Karachi's ethnic-slash-political violence is its complexity. Unlike the one-off reaction to Benazir's death or the terrorist attacks here against foreigners in 2002, the city's indigenous carnage - which has flared up repeatedly since the 1980s - is perhaps the most confounding problem confronting Pakistan's government today. The country suffers from everything from electricity shortages and food and fuel price inflation to stalled talks with India and a total failure to manage natural disasters. But none of these problems present quite the same tangled mix of political incentives that Karachi does.

This city is one of migrants, where Sindhis, Pashtuns, Balochis and Muhajirs - Urdu-speakers arriving from India after 1947 - have uneasily lived in localities that are separated along ethnic lines but crammed next to each other in the 17-million-strong megalopolis. To compete for land, jobs, resources and votes, these groups have formed, or decided to support, rival political parties, which have in turn exploited these ethnic tensions to win supporters.

But thrown into Karachi's mix are also criminal gangs who could not possibly operate so brazenly without the patronage, or at least the blessings, of these parties. Clearly protected, they run land, drug, transport, weapons and extortion rackets and keep rival ethnic groups out of their respective localities; just yesterday, five criminals allegedly belonging to a drug cartel were killed in a shoot-out with law enforcement, who also found arms, ammunitions, and police uniforms in their hideout. But aside from the rare bust such as this one, police and the paramilitary Rangers are famously limited to main roads in many of the city's neighborhoods, whose inner streets are controlled by arms-bearing gang members operating beyond the reach of the law. Every once in a while, sparked by events such as upcoming local elections or high profile assassinations or simply when one group is angry enough at the other, the tense stand-off bursts into flames. This occurred several times in 2009, in the early part of this year, and again this week.

Untying this knot is all but impossible - even before one overlays the city's Sunni-Shia rivalry, politicized and armed through groups such as the Sunni Tehreek (different from the Tehrik-i-Taliban Pakistan, or TTP) and Sipah-e-Sahaba - because the political parties involved are very much a part of Pakistan's governing structure. Between them, the Pakistan Peoples Party, the MQM and the Awami National Party run the federation and three of the four provinces. They are also three of the parties believed to be behind Karachi's ethnic violence over the years.

What this means is that it is incredibly difficult to hold anyone to account for the city's instability. The people at the top of the pyramid are simply too powerful; they city's politics are run by some of the same people who also run the country. And the links between political parties and crime - or at least the desire to exploit rather than curb it - has been disguised well enough so that it is all too easy for a political leader or party to deny their link to the violence; failure to try captured suspects, for example, can easily be attributed to lack of evidence rather than institutional unwillingness to try criminals. It is also possible that leaders at the highest levels of city and provincial politics cannot control the political workers and gangs they have spawned. In any case, under the usual scenario parties find a way of fighting without publicly dragging each other through the mud, perpetuating a cycle that nonetheless allows each of them to maintain their partial hold over Karachi. After a spate of violence in January, for instance, MQM and PPP leaders held a press conference to deny that those clashes - which had claimed at least 35 lives in four days - had ethnic or political roots. They attributed it to "criminals" instead, which is not untrue but is hardly the whole truth.

But Haider's murder, and the lower intensity conflict that preceded it for several weeks, broke away from this mutual public politeness. The MQM and the ANP, locked in clashes that had erupted from land disputes, openly blamed each other. Once Haider was dead, the MQM accused the ANP of at least indirectly being responsible by harboring criminals, extremists, and the Taliban. Meanwhile, Interior Minister Rehman Malik of the PPP insisted that it was a sectarian killing carried out by extremist Sunni group Sipah-e-Sahaba, a version also proposed by the official investigation team.

So even when blame is clearly assigned, the complexity of Karachi's violence protects the people who perpetuate it. Partly through its own evolution and partly through strategy, the situation today is such that that you can throw any number of explanations at it, and most of them will stick - but without holding anyone truly accountable. At any given point, Karachi's violence can be attributed to Islamist militants, criminals peddling various contraband goods or people fighting land grabbers. Everyone but the political parties who could stop the fighting if they wanted to.  

Madiha Sattar is a senior assistant editor at the Karachi-based monthly The Herald.