In the March raid on Egypt's state security agency, Israa Abdel Fattah, a 32-year-old pro-democracy activist who had been jailed twice in the past three years, was shocked to discover in the agency's files copies of her emails, transcripts of phone calls and text messages, and a list of companies where she had applied for jobs.

WSJ: Mideast Uses Western Tools to Battle the Skype Rebellion

Armed with information about where the messages originated, the FBI is believed to have drawn up a list, as far as was possible, of who was at those locations when messages were sent.

BBC: Email trail helped FBI crack Petraeus case