Even so, Ahearn concedes that the uptick in successful terrorism stings might not be evidence of a growing threat so much as a greater focus by the FBI. "If you concentrate more people on a problem," Ahearn says, "you'll find more problems." Today, the FBI follows up on literally every single call, email, or other terrorism-related tip it receives for fear of missing a clue.

And the emphasis is unlikely to shift anytime soon. Sting operations have "proven to be an essential law enforcement tool in uncovering and preventing potential terror attacks," said Attorney General Eric Holder in a December 2010 speech [45] to Muslim lawyers and civil rights activists. President Obama's Department of Justice has announced sting-related prosecutions at an even faster clip than the Bush administration, with 44 new cases since January 2009. With the war on terror an open-ended and nebulous conflict, the FBI doesn't have an exit strategy.

Located deep in a wooded area on a Marine Corps base west of Interstate 95—a setting familiar from Silence of the Lambs—is the sandstone fortress of the FBI Academy in Quantico, Virginia. This building, erected under J. Edgar Hoover, is where to this day every FBI special agent is trained.

J. Stephen Tidwell graduated from the academy in 1981 and over the years rose to executive assistant director, one of the 10 highest positions in the FBI; in 2008, he coauthored the Domestic Investigations and Operations Guide, or DIOG [46] (PDF), the manual for what agents and informants can and cannot do.

A former Texas cop, Tidwell is a barrel-chested man with close-cropped salt-and-pepper hair. He's led some of the FBI's highest-profile investigations, including the DC sniper case and the probe of the 9/11 attack on the Pentagon.

On a cloudy spring afternoon, Tidwell, dressed in khakis and a blue sweater, drove me in his black Ford F-350 through Hogan's Alley [47]—a 10-acre Potemkin village with houses, bars, stores, and a hotel. Agents learning the craft role-play stings, busts, and bank robberies here, and inside jokes and pop-culture references litter the place (which itself gets its name from a 19th-century comic strip). At one end of the town is the Biograph Theater, named for the Chicago movie house where FBI agents gunned down John Dillinger [48] in 1934. ("See," Tidwell says. "The FBI has a sense of humor.")

Inside the academy, a more somber tone prevails. Plaques everywhere honor agents who have been killed on the job. Tidwell takes me to one that commemorates John O'Neill, who became chief of the bureau's then-tiny counterterrorism section in 1995. For years before retiring from the FBI, O'Neill warned [49] of Al Qaeda's increasing threat, to no avail. In late August 2001, he left the bureau to take a job as head of security for the World Trade Center, where he died 19 days later at the hands of the enemy he'd told the FBI it should fear. The agents he had trained would end up reshaping the bureau's counterterrorism operations.

Before 9/11, FBI agents considered chasing terrorists an undesirable career path, and their training did not distinguish between Islamic terror tactics and those employed by groups like the Irish Republican Army. "A bombing case is a bombing case," Dale Watson, who was the FBI's counterterrorism chief on 9/11, said in a December 2004 deposition. The FBI also did not train agents in Arabic or require most of them to learn about radical Islam. "I don't necessarily think you have to know everything about the Ku Klux Klan to investigate a church bombing," Watson said. The FBI had only one Arabic speaker [50] in New York City and fewer than 10 nationwide.

But shortly after 9/11, President George W. Bush called FBI Director Robert Mueller to Camp David. His message: never again. And so Mueller committed to turn the FBI into a counterintelligence organization rivaling Britain's MI5 in its capacity for surveillance and clandestine activity. Federal law enforcement went from a focus on fighting crime to preventing crime; instead of accountants and lawyers cracking crime syndicates, the bureau would focus on Jack Bauer-style operators disrupting terror groups.

To help run the counterterrorism section, Mueller drafted Arthur Cummings, a former Navy SEAL who'd investigated the first World Trade Center bombing. Cummings pressed agents to focus not only on their immediate target, but also on the extended web of people linked to the target. "We're looking for the sympathizer who wants to become an operator, and we want to catch them when they step over that line to operator," Cummings says. "Sometimes, that step takes 10 years. Other times, it takes 10 minutes." The FBI's goal is to create a hostile environment for terrorist recruiters and operators—by raising the risk of even the smallest step toward violent action. It's a form of deterrence, an adaptation of the "broken windows" theory used to fight urban crime. Advocates insist it has been effective, noting that there hasn't been a successful large-scale attack against the United States since 9/11. But what can't be answered—as many former and current FBI agents acknowledge—is how many of the bureau's targets would have taken the step over the line at all, were it not for an informant.

So how did the FBI build its informant network? It began by asking where US Muslims lived. Four years after 9/11, the bureau brought in a CIA expert on intelligence-gathering methods named Phil Mudd [51]. His tool of choice was a data-mining system using commercially available information, as well as government data such as immigration records, to pinpoint the demographics of specific ethnic and religious communities—say, Iranians in Beverly Hills or Pakistanis in the DC suburbs.

The FBI officially denies that the program, known as Domain Management, works this way—its purpose, the bureau says, is simply to help allocate resources according to threats. But FBI agents told me that with counterterrorism as the bureau's top priority, agents often look for those threats in Muslim communities—and Domain Management allows them to quickly understand those communities' makeup. One high-ranking former FBI official jokingly referred to it as "Battlefield Management."

Some FBI veterans criticized the program as unproductive and intrusive—one told Mudd during a high-level meeting that he'd pushed the bureau to "the dark side." That tension has its roots in the stark difference between the FBI and the CIA: While the latter is free to operate internationally without regard to constitutional rights, the FBI must respect those rights in domestic investigations, and Mudd's critics saw the idea of targeting Americans based on their ethnicity and religion as a step too far.