In James Bamford's first book on the National Security Agency, The Puzzle Palace , published soon after Reagan became President, Frank Raven, an NSA official, is asked what happens when someone on whom the NSA is spying enters the US. 'You have intelligence which is entirely foreign and you have intelligence which is entirely domestic,' Raven says. 'But then you have the third category which no one will recognise, which is intelligence which moves back and forth between them.' Twenty years later, another NSA official, quoted in Body of Secrets, explains what would happen if a member of al-Qaida crossed the American border. 'We wouldn't do the guy. It would be FBI who'd do him, because he's a terrorist in the United States.' On the one hand, the NSA, trained to pluck Soviet transmissions from the ether: on the other, the FBI, with its experience of domestic manhunts. Free to operate in the space left between the two are men who are neither official agents of a hostile foreign government nor homegrown criminals.