There really is nothing that the government isn't capable of if they want to do something badly enough. There are enough people willing to look out for their own, and not willing to expose the corruption that the government is powerful enough to keep anything a secret, away from the public's view.
In recent times, we have seen the violation of basic human rights by the U.S. government, including things like spying on private and personal internet information. It is not entirely unreasonable to think that such acts could happen again, or that they are happening now without any knowledge or consent.
In south carolina for example they sprayed for mosquitos and killed millions of bees. There are lots of other small examples, and it is entirely possble that something even bigger could happen. The US is run by a bunch of functional idiots and it would be relatively easy to pull off.
The United States does all kinds of things that it doesn't tell its citizens about. There were all of those attacks on Fidel Castro that were never successful. The United States has entire departments such as the CIA and the FBI that are devoted to espionage. The American people have no idea the risks they are under at the mercy of their own government.