AT&T and the NSA are not the same thing.

AT&T is almost certainly not storing every text message long-term. The NSA likely is. It wouldn't be surprising if that's via a direct PRISM-style integration, but it still means subpoenaing AT&T for old texts is likely to not be productive. (As it was not productive in this particular case.)

Subpoena the NSA and they'll say "no, for national security reasons".

I'm not even sure we can extrapolate that the NSA is storing anything like every text message long-term (as opposed to metadata, or samples, or the full transcripts of individuals targeted for investigation, or a machine learning trained weights set to flag messages that indicate someone should go on the targeted-for-investigation list).

6 billion text messages are sent per day in the US. That's about the volume of Google web searches, and I know from experience Google doesn't have the capacity to log every search or the logs of evaluation of every search. If Google lacks the capacity, I suspect the NSA lacks the capacity.

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