It's more like the FBI's investigations into various criminal and subversive groups.
The FBi is a law enforcement organization. It's their job to investigate US citizens when there is reason to believe they are involved in breaking US law.
The NSA is a military organization. Their charter (and the constitution) explicitly precludes them from targeting US citizens, yet they do it anyway. What does it mean when a country's military deems every citizen such a threat to national security that they are considered valid intelligence targets? It says to me we are considered the enemy - each and every one of us. This cannot possibly end well.