FBI counterintelligence specialist Robert Hanssen was a devout Catholic and family man whose professional diligence resulted in him being trusted with the highest possible security clearance. Yet last February, he was charged with spying for the Russians during most of the past 15 years, passing over 6,000 top-secret documents in return for diamonds and cash. Tom Mangold investigates the background to what may be the west's biggest spying case since Kirn Philby.