“Four weeks on, the world still does not know exactly what legal basis the US government used to obtain a secret court order requiring Yahoo! to scan all of its users’ incoming e-mails for a string of characters the government said was associated with a terrorist group. While sources have suggested that the court issued its order under a certain provision of the Foreign Intelligence Surveillance Act, the government has refused to confirm this publicly. Additionally – despite a plea from Yahoo! – the government has so far refused to declassify the order, a stance it reiterated to Reuters this week.
This silence cannot be squared with the human rights laws that are binding upon the United States.”
See: US Still Dodging Questions On Yahoo! Spying (HRW, link)