Plausible deniability began under Central Intelligence Agency (CIA) director Allen Dulles. The CIA black ops division undertakes dangerous and usually what would be considered illegal missions that are not officially sanctioned by the US administration so that the administration, which usually benefits from such missions, can safely disavow any knowledge of them in the event of their publically uncovered success or failure. The administration is in the position of plausible deniability towards the CIA's actions.
The Snowden leaks revealed a massive surveillance program including interception of email and other Internet communications and phone call tapping. Some of it appears illegal, while other documents show the US spying on friendly nations during various international summits, and on its citizens. The programs are enabled by two US laws, the Patriot Act and the FISA Amendments Act (FAA), and a side dish called Executive Order 12333. Although the Foreign Intelligence Surveillance Court oversees such surveillance activities, it operates in secrecy through one-sided procedures that favour the US government.
Executive Order 12333 - 1.13 also allows the FBI to provide the NSA with “technical assistance” in the United States, i.e. to collect metadata about American citizens by collaborating with “foreign intelligence” and law enforcement services. By using telecom companies the US government can circumvent controls and accountability that don't apply to companies. All major telecom companies have worked closely and secretly with government spying since their inceptions.
In 2012, the NDAA, the National Defense Authorization Act was changed from only applying to “Enemy Combatants” to American citizens themselves. The Act allows for U.S. citizens in addition to enemy combatants to be arrested on American soil and to be detained for an indefinite amount of time.