Pro Publica Questions Claims of NSA Spying Stopping Terror Attacks

Important piece just posted by ProPublica on those claims by President Obama and others that various forms of NSA spying have "thwarted" over 50 terror attacks--the exact number is usually given as 54--a number I've questioned (without clear evidence) for a long time.  Now ProPublica writers say,   "there's no evidence that the oft-cited figure is accurate.  The NSA itself has been inconsistent on how many plots it has helped prevent and what role the surveillance programs played. The agency has often made hedged statements that avoid any sweeping assertions about attacks thwarted....Asked for clarification of the surveillance programs' record, the NSA declined to comment."