Whether the topic is ex-national security adviser Michael Flynn's leaked contacts with Russian Ambassador to the US Sergey Kislyak, Trump's unsubstantiated claim that former President Barack Obama wiretapped Trump Tower or House Intelligence Committee Chairman Devin Nunes' recent pronouncement that some of Trump's communications may have been swept up, to understand the story, one must understand the terms involved.
The National Security Agency, or NSA, is a US intelligence organization primarily tasked with the collection and analysis of foreign signals intelligence -- communications, generally through electronic means, be it a phone call, an email or something else entirely. This can also include metadata -- the information about the communications themselves -- like when, where and to whom a message is sent or received.
The NSA's mandate is to deal with foreign intelligence targets, like militants overseas or members of another government.
It is headquartered in Maryland, but its presence is felt globally, scooping up and processing information from people located virtually anywhere.
Its activities are covert, wide-ranging and linked to any number of US security efforts.
NSA has wide discretion to conduct surveillance on people outside of the US, but there are supposed to be legal barriers to snooping on US citizens. If the NSA picks up communications from a US citizen in the course of monitoring a foreign national, it is practice to "mask" the identity of the US person. Additionally, the identification of a US citizen mentioned in discussions between two foreign nationals is supposed to be masked.
But if the NSA or another agency with which NSA is sharing the information, like the FBI, wants to identify the person, it can do so if it believes it's necessary in the course of investigations or have probable cause to indicate there may be criminal conduct involved. Rogers said the number of people who have unmasking authority was limited in the NSA to about 20 people, while FBI Director James Comey said there were necessarily many more in the FBI who could do so because the agency's work involves domestic affairs.
Nunes said Trump's communications and those of people involved in his campaign may have been collected by the government through "incidental collection."
That means that in the course of monitoring for foreign intelligence purposes, the government may have caught the private communications of then President-elect Trump. But as noted later Wednesday by Democratic Rep. Adam Schiff, the ranking member on the House Intelligence Committee, incidental collection could refer simply to the mention of a US citizen, not that their communications were collected.
"If we are listening to two foreign spies, for example, talking to each other on foreign soil or two representatives of a foreign government, and they mention a US person -- that is incidental collection," Schiff said.
Passed alongside nine other amendments together known as the Bill of Rights, the Fourth Amendment is meant to curtail "unreasonable" searches from the government and require warrants based on probable cause for searches.
It is the main constitutional right that the US surveillance state butts against.
The Foreign Intelligence Surveillance Act of 1978 established the Foreign Intelligence Surveillance Court (FISC), a secretive court that can approve or disapprove spying requests. Its authority grew following the Sept. 11 terrorist attacks.
Former NSA contractor Edward Snowden leaked documents in 2013 exposing US surveillance activities and techniques, leading to the passage of the USA Freedom Act in 2015. The law implemented a set of reforms to FISC, which as of 2016 had rarely turned down requests from the government.
Additionally, FISA is often used not just to refer to the law itself but authorized surveillance under it. Similarly, the terms FISA orders, FISA warrants, etc. are used regularly.
In order to invade a US citizen's privacy for a search or seizure, law enforcement is legally required to have a warrant.
Laws passed following the revelation have attempted to narrow the scope of warrantless wiretaps.
Section 702 of FISA is set to expire at the end of this year unless Congress votes to renew it and Trump signs that renewal into law.
It was one of the changes to FISA introduced in 2008 and establishes rules for the Justice Department and the director of national intelligence together target people located outside the US.
Government Communications Headquarters is essentially the equivalent to the NSA in the United Kingdom.
The United States has an intelligence alliance with four other English-speaking nations: The United Kingdom, Canada, Australia and New Zealand.
The five nations share intimate intelligence as part of what they call the "Five Eyes" agreement.
In his appearance before the House Intelligence Committee on Monday, Rogers said asking GCHQ to wiretap Trump would be at odds with the Five Eyes agreement.
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'Unmasking,' FISA and other terms to help you understand the wiretapping story - CNN