It's not just the NSA: A Federal Appeals Court has just noted a disturbing and "extraordinary" trend of the Navy conducting mass surveillance on American civilians, and then using what they find to help local law enforcement prosecute criminals.
In this specific case, a Navy Criminal Investigative Service agent in George scanned the computers of every civilian in Washington state who happened to be using the decentralized Gnutella peer-to-peer network, looking for child pornography. The agent, Steve Logan, found child porn on a computer owned by a man named Michael Dreyer.
Logan then passed his evidence on to local law enforcement, who arrested and eventually convicted Dreyer, who was sentenced to 18 years in prison. The US Ninth Circuit of Appeals ruled that this was a massive overstep of military authority, a disturbing trend, and a blatant violation of the Posse Comitatus Act, a law that prohibits the military from conducting investigations on civilians.
The government argued that it conducted the surveillance on the off chance that it caught a military member violating the law and suggested that it has this authority in any state with a military base.
For the rest of the story: http://motherboard.vice.com/read/the-navy-routinely-spies-on-citizens-then-helps-the-police-prosecute-them