Probably impossible to verify one way or the other. May be more true to say "...have led to little evidence of stopping...". See, e.g., Schneier's article on this (https://www.schneier.com/blog/archives/2014/01/how_the_nsa_t...), which references a summary article on what may be true about the effectiveness of the surveillance in stopping plots/attacks (http://www.huffingtonpost.com/2013/10/23/nsa-attacks-thwarte...).