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  (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 —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  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  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  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.