[readitalready]

NSA legally isn't allowed to spy on US citizens directly, due to the NSA being a US military organization and the Posse Comitatus act prohibits the US military from being used as a US policing force.

It's one of the hidden and forgotten revelations about the Snowden leaks, where he showed that the NSA had a bunch of filters in their top-secret classified systems to filter out communications from US citizens. Those filters exist because of Posse Comitatus.

[chirau]

How does the filter work? Identity first? As in, do they access the data/activity first and stop when they realize the person is a citizen? Otherwise how do they approach it?