Q:

Have the NSA spying programs stopped any terrorist attacks?

The government claims that these security programs are necessary for our safety, and have cited over 50 examples wherein terrorist plots were discovered and stopped due to this surveillance. However, opponents have debunked at least some of these claims, which have been either exaggerated, or were primarily stopped via traditional law enforcement methods.
Are there any publicly known cases of a credible terrorist attack that was discovered or stopped because of large-scale wiretapping/traffic analysis/etc. playing an essential role, without which the plot would not have been discovered or stopped?
For the purposes of answering, if a plot was initially discovered by one of these surveillance programs, it will be assumed that it would not have been discovered otherwise, unless there is other evidence indicating that there was another lead from elsewhere.

A:

As far as I know there has only been one successful "plot" foiled, involving a San Diego cab driver who gave $8,500 to an al-Qaeda affiliate(archive.org version). Note that the source may have some bias even though it claims to be nonpartisan. 

