It is a mark of Karachi's cosmopolitanism that most of its millions of citizens carry on life as if this underworld does not exist. In that regard, it is in many ways no different from any other 21st century metropolis -- ungainly, exciting, raucous, difficult. There are hip clubs with DJs, cool new restaurants with enigmatic names, a burgeoning middle class. Kids ride bikes, markets hawk DVDs and digital cameras, the bright shiny silks of upscale ladies-about-town billow in the breeze. It is Pakistan's most progressive city, former home to its first female prime minister, Benazir Bhutto, and thousands of women go about the city unescorted, unveiled, running errands, going to jobs, lunch dates, and prenatal classes. Graduates of its university engineering programs are prized in technology centers and other outposts of the new world economy.

Still, especially after the 2002 murder of Wall Street Journal reporter Daniel Pearl, it seemed a woolly, scary place to do business. Agents routinely felt they needed to run what they called surveillance detection routes, SDRs, when they went to and from their living quarters.

Raids, even when they were able to mount them, didn't seem to produce much. Karachi was terribly overbuilt. Much of the construction industry was controlled by the military, and much of the military's money was illicit. It couldn't just sit around; it had to be put to use. It built buildings whether the market existed for them or not. So even a city that was growing an average of 5 percent a year had a perpetually high rate of vacant buildings. It made Karachi an easy place to hide. You could slip in and out of empty places -- a new one every day if you wanted; you could rent them for almost nothing. A series of raids in the spring and summer of 2002 had found a lot of empty flats.

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In August, the FBI caught a break when it questioned a brother-in-law of KSM, Abdul Samad Din Muhammad, who had been arrested and questioned in the United Arab Emirates in November 2001 and extradited to Pakistan in 2002. Muhammad told FBI agents that Aziz Ali was in constant contact with his uncle, KSM. He also said Aziz Ali received a constant stream of Arab visitors from Pakistan at the airport and that Ali had suddenly bolted from the UAE a day or two before the Sept. 11 attacks. He didn't have his belongings together, but insisted on leaving. When Muhammad asked Aziz Ali why he was in such a rush to leave, he didn't get a satisfactory answer. FBI deputy legal attaché Jennifer Keenan, who was working closely on the case, was now certain that the way to get to KSM was through his nephew.

More raids initially yielded nothing, but in early September, the Pakistani police got lucky. Neighbors had pointed out that there was an awful lot of traffic through a house in the Gulshan-e-Iqbal neighborhood. Police nabbed a man leaving the house on his way to pay utility bills. Agents of the ISI investigated and detained the man, a Saudi native, who said he managed the house. His name was Mohammed Ahmad Rabbani. Rabbani's driver proved to be quite talkative. He said Rabbani and his brother managed several similar guesthouses, all of which had a constant stream of guests. He helpfully gave police the addresses of the houses.