Whodunnit?

VIOLENCE in Pakistan comes in several flavours. There is sectarian violence between Sunni and Shia Muslims, whose signature is the killing of worshippers in their mosques. There is internecine carnage among mohajirs—Pakistanis whose families emigrated from India—which not long ago had turned Karachi, the main commercial city, into a no-man's land at night. The bomb that killed 16 people in a vegetable market on the outskirts of Islamabad, the capital, on September 19th, is more mysterious. Its victims were chosen at random; the motives of its perpetrators are unknown.