Did the “thing” mentioned on the wiretaps really refer to money for the Taliban? Illustration by Mike McQuade

At dawn on May 14, 2011, more than two dozen federal agents and local police officers converged on a working-class neighborhood near the Miami airport and surrounded a small green-and-white stucco building—Masjid Miami, one of the city’s oldest mosques. Police sealed off a two-block radius, and F.B.I. agents, some armed with AR-15 rifles, assembled outside the door.

Inside, eight men were kneeling for the first prayer of the day. When agents called for them to open up, one of the worshippers, a former police officer, went out and asked them to wait until the prayer was finished. The agents complied, and then they arrested the mosque’s imam, Hafiz Khan, an émigré from a mountainous corner of Pakistan near the Afghan border. Khan was in his late seventies, an albino with thick glasses and a long colorless rush of beard. He had moved to America, with members of his family, in 1994, at the encouragement of a younger brother in Alabama. They became citizens, but Khan spoke no English and rarely left the mosque or a one-room apartment across the street, which he shared with his wife, Fatima. He was known to some of the locals as el viejito barbón—the old bearded man. Kids referred to him as the Santa Claus imam.

While the F.B.I. was arresting Khan, another team of federal agents and police assembled forty miles away, in the city of Margate. They surrounded Jamaat Al-Mu’mineen, a large mosque presided over by Hafiz’s youngest son, Izhar Khan. Izhar, who was twenty-four, was about to lead the morning prayer when agents in F.B.I. windbreakers confronted him in the parking lot. Izhar had moved to Florida when he was eight years old, and he spoke barely accented English. He wore a long dark beard, a black cotton robe, and a skullcap. The agents examined the computers in his office, and when they searched his cell phone they noticed that many of his text messages were about the Miami Heat and other teams.

Meanwhile, a third maneuver in the F.B.I.’s operation against the Khans was unfolding in Los Angeles, where it was 3 A.M. and Izhar’s brother Irfan, a thirty-seven-year-old software programmer, was asleep in his room at the Homestead Studio Suites, an inexpensive business hotel in El Segundo. Married, with two kids, Irfan was a sitcom buff who made hammy jokes about his waistline. (“This won’t be good for my diet!”) He lived in Miami and worked for American Unit, an I.T. company. For the past three months, he had been commuting every two weeks to an assignment in El Segundo. He was awakened by a phone call from the police, advising him to go to the door. He was handcuffed and led to a waiting car, past bomb-sniffing dogs and helmeted men in camouflage.

After the arrests, federal authorities announced that, in all, six people in Florida and abroad had been charged with funnelling tens of thousands of dollars into a conspiracy to “murder, kidnap, or maim persons overseas,” orchestrated by the Pakistani Taliban, an ally of Al Qaeda. The group was known for having trained Faisal Shahzad, a Pakistani-American who, in May, 2010, tried to set off a car bomb in Times Square. In 2012, Pakistani Taliban gunmen boarded a bus in northwest Pakistan and shot Malala Yousafzai, a fifteen-year-old schoolgirl who had called for the education of women.

The F.B.I. had been secretly tracking the Khans for at least a year, monitoring their finances and recording thousands of hours of conversation, in person and on the phone. Two other members of the family were also indicted—a daughter and a seventeen-year-old grandson, who live in Pakistan—along with a Pakistani shopkeeper, who had served as a middleman. In the indictment, they were accused of conspiring to buy guns, shelter the Taliban, and send students “to learn to kill Americans in Afghanistan.” The indictment described phone calls from Miami, in which the father “called for an attack on the Pakistani Assembly” and “called for the death of Pakistan’s President.” The U.S. Attorney Wifredo A. Ferrer told the Sun Sentinel that a list of cash transfers totalling fifty thousand dollars was “just the tip of the iceberg,” and declared, “We will be able to prove that there is more than fifty thousand dollars that went to the Taliban.” Each of the accused faced between forty-five and sixty years in prison.

The process that landed the Khans in court began, in effect, shortly after the September 11, 2001, attacks, when John Ashcroft, the Attorney General, sent a memo to senior law-enforcement officials declaring a new focus: “proactive prevention and disruption.” It marked a fundamental shift, from pursuing terrorists after the fact to preëmpting them, by arresting their supporters as early as possible, often with the help of informants. In a 2003 bulletin to U.S. Attorneys, a Department of Justice official identified laws that allow “strategic overinclusiveness.” In those cases, the connections to violence can be attenuated: in the Khan case, prosecutors charged the defendants with, among other things, conspiring to conspire—intending to support others who, in theory, intended to engage in terrorism.

The growing use of informants in the campaign against terrorism has drawn criticism from lawyers and activists, and even from some judges; they say that the F.B.I. has used informants to manipulate people who are vulnerable because of mental illness or financial desperation. In a terrorism case in 2011, known as the Newburgh Four, Judge Colleen McMahon faulted the F.B.I. for “trolling among the citizens of a troubled community, offering very poor people money if they will play some role, any role, in criminal activity.” In that case, an informant in Newburgh, New York, offered four men a quarter of a million dollars, a BMW, and other rewards to shoot down military planes, and to set off explosives at two synagogues in the Bronx. They joined the plot. The judge called their actions “beyond despicable,” but, referring to the ringleader, she said that the bureau had manufactured a terrorist out of a former Walmart employee whose “buffoonery is positively Shakespearean in its scope.” (She rejected the prosecutor’s request for life sentences, and sent the accused to prison for twenty-five years, the statutory minimum.)

The case against the Khans rested on the two most frequently used tools in the legal battle against terrorism: conspiracy and “material support.” Under material-support laws, it is a felony to knowingly provide money, shelter, or technical advice to anyone involved with terrorism. Before 2001, the Justice Department had used that charge on only six occasions; in the first three years after 9/11, the department filed the charge nearly a hundred times, but then its use subsided. Recently, the pace has accelerated again. This year, the Justice Department has filed material-support charges against at least fifty-seven defendants who are accused of allying with the Islamic State. The Khan-family case bore many hallmarks of America’s legal battle with terrorism, but counterterrorism officials rarely set out to capture what a prosecutor called “an entire family that has participated in extreme violence.” Rarer still does a case evolve in such a way that, once it’s over, the family tells its story.

The Khans came from the Swat Valley, a fertile corridor amid the mountains of the Hindu Kush. Since the Soviet invasion of Afghanistan, in 1979, Swat has seen a series of conflicts among governments, Islamic radicals, and local residents. Pashtuns, the dominant ethnic group in Swat, have been embroiled in the violence: in Pakistan and Afghanistan, more than a million have been killed, and millions have been displaced.