Barton Gellman's informant called himself 'Verax' - the truth-teller. It was only later that Verax unmasked himself as Edward Snowden. But Gellman's primary role in bringing Snowden's revelations to light, for which he shared the Pulitzer prize, is only the beginning of this gripping real-life spy story. Snowden unlocked the door: here Gellman describes what he found on the other side over the course of a years-long journey of investigation. From a Pulitzer Prize winning journalist.