In short, the NSA was not merely conducting illegal spying on Americans, but it was systematically perusing millions of electronic communications to identify people who might conceivably, based on very gross indicators, be sympathetic or supportive of activities that might remotely be hostile to the United States. The swath is so wide that the net is more likely to scoop up vast schools of innocents than the small numbers of people who really mean to do harm and violence.
The New York Review of Books: 'The Biggest Secret': "So why did the administration continue this lumbering effort for three years? Outsiders sometimes find it tempting to dismiss such wheel-spinning as bureaucratic silliness, but I believe that the Judiciary Committee will find, if it is willing to persist, that within the large pointless program there exists a small, sharply focused program that delivers something the White House really wants. This it will never confess willingly."