Strictly speaking it's been the FBI doing the surveillance itself and turning the data over for storage to NSA, if I understand the org. structure correctly.

I must admit I haven’t read the articles about Snowden’s discloures that close. Do you have any links to share regarding the FBI:s involvement?

The original WaPo/Guardian articles on PRISM are a good start, and I think one of the followup NYT articles was even more specific about how the NSA manages to capture information from domestic providers even though they're legally not allowed to intercept domestically.

Instead they have FBI install data collection devices, have the FBI runs NSLs/warrants as needed, etc. NSA just does the spook work after all that is done.

Thanks. Ever since I read Clifford Stoll’s ”The Cuckoo’s Egg” I’ve been under the impression that the NSA and the FBI hated each other :)

Well that pretty much was the case, right up until 9/11. ;)