The recent revelation that Verizon had turned over phone records of U.S. citizens to the National Security Agency (NSA) in compliance with a top secret court order is only the most recent incident shedding light on the activities of the secretive, surveillance agency.

Information about the intelligence agency, falling under the Department of Defense, has been leaking out as it has been evolving for the last 60 years.

A History of NSA and Its Spying Activities

NSA’s headquarters in Fort Meade, Md. (Image: NSA via Wikimedia)

1950s: The NSA was established on Nov. 4, 1952, by President Harry Truman — preceded by the Armed Forces Security Agency, which was established in 1949 – to help break German and Japanese codes during WWII. It settled its headquarters at Fort Meade, Md.

1970s: Central Security Service was established to work with the NSA on missions regarding SIGINT (signal intelligence, collecting information that America’s enemies would “want to keep secret”) and information assurance (keeping America’s information safe).

A History of NSA and Its Spying Activities

A sign stands outside the National Security Administration (NSA) campus in Fort Meade, Md. , Thursday, June 6, 2013. The Obama administration on Thursday defended the National Security Agency’s need to collect telephone records of U.S. citizens, calling such information “a critical tool in protecting the nation from terrorist threats.” (Photo: AP/Patrick Semansky)

Washington Post journalist Bob Woodward reveals the NSA intercepted conversations of high-profile U.S. citizens, especially “anti-war leaders.” Communications that were intercepted include overseas cables, domestic telegrams and long-distance calls.

1980s: According to the Washington Times, the NSA recently declassified some of its history, which revealed the square footage of its facilities increased and the agency’s employee base rose 40 percent in this decade. The Times estimated NSA’s workforce to be between 30,000 to 40,000 employees.

1990s: United States Signals Intelligence Directive 18 established policies to “ensure that the missions and functions of the United States SIGINT System (USSS) are conducted in a manner that safeguards the constitutional rights of U.S. persons.” Section 4 establishes the conditions and how the NSA could intercept communications of U.S. citizens.

2000s: Then NSA director, Lt. Gen. Michael Hayden responded to allegations regarding the NSA’s electronic surveillance of U.S. citizens and its safeguards. George Washington University stated of the event:

The agency may only target the communications of U.S. persons within the United States after obtaining a federal court order suggesting that the individual might be “an agent of a foreign power.”  The number of such cases have been “very few” since the passage of the Foreign Intelligence Surveillance Act in 1978.  In cases where the NSA wishes to conduct electronic surveillance on U.S. persons overseas, the agency must first obtain the approval of the Attorney General, who must have probable cause to believe that the individual “is an agent of a foreign power, or a spy, terrorist, saboteur, or someone who aides or abets them.”  With regard to the unintentional collection of communications to, from, or about U.S. citizens, Hayden stresses that such information is not retained “unless the information is necessary to understand a particular piece of foreign intelligence or assess its importance.”

Several whistleblowers, including some NSA employees, began revealing the extend of the NSA’s spying on communications of U.S. citizens. Two former NSA employees, William Binney and J. Kirk Wiebe, resigned after they believed a system they developed (ThinThread) was being used to survey American’s communications. Here’s more on their whistleblowing from Whistlebower.org:

Since that time, Binney and Wiebe have made several key disclosures crucial to the ongoing public debate about America’s national security state, such as the first public description of NSA’s massive domestic spying program, Stellar Wind, which intercepts domestic communications without protections for US citizens. Binney revealed that NSA has been given access to telecommunications companies’ domestic and international billing records, and that since 9/11 the agency has intercepted between 15 and 20 trillion communications. Binney further disclosed that Stellar Wind was grouped under the patriotic-sounding “Terrorist Surveillance Program” in order to give cover to its constitutionally-questionable nature.

In 2006, a former AT&T employee revealed that the NSA had built monitoring equipment in “secret rooms” at AT&T’s San Francisco switching office:

In January 2003, I, along with others, toured the AT&T central office on Folsom Street in San Francisco—actually three floors of an SBC building. There I saw a new room being built adjacent to the 4ESS switch room where the public’s phone calls are routed. I learned that the person whom the NSA interviewed for the secret job was the person working to install equipment in this room. The regular technician work force was not allowed in the room.

In 2012, author and journalist James Bamford revealed Binney saying both AT&T and Verizon were part of Operation Stellar Wind. Here’s what Bamford wrote of the operation:

According to Binney, one of the deepest secrets of the Stellar Wind program—again, never confirmed until now—was that the NSA gained warrantless access to AT&T’s vast trove of domestic and international billing records, detailed information about who called whom in the US and around the world. As of 2007, AT&T had more than 2.8 trillion records housed in a database at its Florham Park, New Jersey, complex.

Verizon was also part of the program, Binney says, and that greatly expanded the volume of calls subject to the agency’s domestic eavesdropping. “That multiplies the call rate by at least a factor of five,” he says. “So you’re over a billion and a half calls a day.” (Spokespeople for Verizon and AT&T said their companies would not comment on matters of national security.)

Bamford also provided details on the still secretive Utah Data Center, which is expected to complete construction in Bluffdale, Utah, fall 2013. He called the amount of data that $1.9 billion center will be able to store “staggering,” and noted that he expected information passing into it to include “password-protected data, US and foreign government communications, and noncommercial file-sharing between trusted peers.”

A History of NSA and Its Spying Activities

An aerial view of the NSA’s Utah Data Center in Bluffdale, Utah, Thursday, June 6, 2013. The government is secretly collecting the telephone records of millions of U.S. customers of Verizon under a top-secret court order, according to the chairwoman of the Senate Intelligence Committee. The Obama administration is defending the National Security Agency’s need to collect such records, but critics are calling it a huge over-reach. (Photo: AP/Rick Bowmer)

In 2012, it was also revealed to Wired by Sen. Ron Wyden (D-Ore.) that The Foreign Intelligence Court had found NSA “on at least once occasion” violated the Fourth Amendment while collecting intelligence information.

All this — and more — culminates to the latest revelation that the NSA allegedly has been spying on communications of all Verizon customers. Two members of Senate Intelligence Committee also stated that this activity has been going on for the last seven years.

Former NSA employees have alleged that the spy agency collects records on about 3 billion phone calls a day from all major U.S. phone companies.

The NSA and FBI have also been tapping into the servers of 9 top Internet companies to obtain Americans’ documents, photos, audio and other information, according to a shocking report from The Washington Post.

A History of NSA and Its Spying Activities

This photograph made Thursday, July 6, 2013 in Washington shows a copy of the U.S. Foreign Intelligence Surveillance Court order requiring Verizon on an “ongoing, daily basis,” to give the National Security Administration (NSA) information on all landline and mobile telephone calls of Verizon Business in its systems, both within the U.S. and between the U.S. and other countries. (Photo: AP)

Sen. Dianne Feinstein (D-Calif.), while defending the collection of metadata of phone calls, said that as she understood it, “this is the exact three-month renewal of what has been in place for the past seven years,” as carried out by Foreign Intelligence Surveillance Court.