The National Security Agency and the FBI are tapping directly into the central servers of nine leading U.S. Internet companies, extracting audio, video, photographs, e-mails, documents and connection logs that enable analysts to track a person’s movements and contacts over time. The highly classified program, code-named PRISM, has not been disclosed publicly before. Its establishment in 2007 and six years of exponential growth took place beneath the surface of a roiling debate over the boundaries of surveillance and privacy. Even late last year, when critics of the foreign intelligence statute argued for changes, the only members of Congress who know...