It's a claim made by the head of the agency that did it. If you're not willing to take their word, I think you're being stubborn and obtuse and this will never be a productive discussion.
If you're willing to believe a US whistleblower -- whose evidence you can't personally verify -- you have to be open to believing the _former head of the agency_.
The second is based on some yet unreleased documents that Der Spiegel claims to have reviewed years ago.
Still no evidence to be found, only claims.