To protect the classified information stored on her desktop computer, Special Agent Nenette Day uses one of the most powerful tools on the planet-an air gap. 

Day points to an IBM ThinkPad resting on the table behind her desk. "That computer is hooked up to the Internet," she says. "But if you break into it, have a good time: there's no secret work on it."

Two meters away on her desk sits Day's other computer-a gray-and-chrome minitower emblazoned with a red sticker proclaiming that its hard drive is classified SECRET. "This," she says protectively, "holds my e-mail." Day readily talks about the ThinkPad, describing how she got it as part of a big purchase by the Federal Bureau of Investigation (FBI) a few years ago and explaining that it's now somewhat out-of-date. And she happily shows off a collectible action figure-still in its display box-a colleague brought back from Belgium. It's a "cyberagent" with a gun in one hand and a laptop computer in the other. But if you let your eyes drift back to that red sticker and try to copy the bold, black words printed on it, Day will throw you out of her office.

Day belongs to the FBI's Boston Computer Crime Squad, one of 16 such units located throughout the United States. Each is composed of about 15 agents who investigate all manner of assaults on computers and networks-everything from lone-hacker to cyberterrorist attacks-with a dose of international espionage thrown in for good measure. Crimes range from Web site defacements and break-ins to so-called denial-of-service attacks, which prevent legitimate users from accessing targeted networks.