Revelations from US intelligence leaker Edward Snowden have shed a rare public spotlight on the ultra-secret National Security Agency, which uses super computers and code-breaking mathematicians to oversee the world's most powerful electronic spying operation.
Hollywood directors and novelists have made the CIA famous for its undercover agents in the field, but in the digital era, the hi-tech NSA may represent the most far-reaching arm of the country's 16 spy agencies, with its intelligence at the center of decision-making and military planning, experts say.
The latest leak from Snowden alleges the NSA has been eavesdropping on US allies, including the European Union offices in Washington, which would fit in with the agency's ability to scoop up any conversation or email relevant to "foreign targets."
Created after World War II to avoid another Pearl Harbor-style surprise attack, the NSA "has transformed itself into the largest, most covert, and potentially most intrusive intelligence agency ever created," wrote author James Bamford, whose books helped lift the lid on the agency's work.
With code-breaking services in a disorganized jumble, President Harry Truman set up the NSA through a secret directive in 1952, allowing the agency virtually free reign to snoop on the Soviet Union and to track communications entering and leaving the United States.
Employees at the secrecy-minded agency would say they worked at the Defense Department, earning the NSA nicknames such as "No Such Agency" and "Never Say Anything."
While the CIA may break into a building to plant a bug, the NSA is in charge of information "in motion," vacuuming up data transiting telecommunication cables or radio waves.
Congress imposed more oversight and stricter legal guidelines in the 1970s after a Senate inquiry exposed numerous intelligence abuses, including the use of the NSA to spy on Americans involved in anti-war and other protests.
Not only is the NSA in charge of all manner of "signals intelligence," the agency's chief also heads up the military's Cyber Command for digital warfare, and the service plays a vital role in securing computer networks against a cyber attack on the United States.
The NSA's budget remains classified but it is believed to be the largest in the intelligence community. Funding doubled since the attacks of September 11, 2011, according to the book "Top Secret America" by journalists Dana Priest and William Arkin.
The agency's sprawling headquarters in Fort Meade, Maryland, northeast of Washington, has its own exit on the freeway for employees only. The number of NSA workers is also a secret, though one top official once joked the workforce was somewhere between 37,000 and a billion.
Since the advent of the Internet and the demand for intelligence on Al-Qaeda after 9/11, the NSA has steadily grown in importance, hiring tens of thousands of contractors, like Snowden, to manage its extensive operations that require cryptologists, linguists, electrical engineers and other technicians.
In its early years, the NSA inherited a program called "Shamrock," in which the agency intercepted up to 150,000 telegraph messages a month, with the help of American companies who agreed to the arrangement despite worries about its legality.
Now, each day "collection systems at the NSA intercept and store 1.7 billion emails, phone calls and other types of communications," and the NSA "sorts a fraction of those in 70 separate databases," according to "Top Secret America."
To store the massive amount of data, the agency is constructing a vast storage center in the Utah desert at a cost of $2 billion, which will serve a computer "cloud" for the NSA.
The NSA's alleged spying on US allies is not the first time Washington has been accused of snooping on friendly governments to gain an edge in diplomacy and trade.
In 2000-2001, members of the European parliament were outraged after revelations of an elaborate eavesdropping operation carried out by the US, Britain, Canada, Australia and New Zealand, which they alleged had been used to undermine some European firms.
And in the 1920s, code-breakers at NSA's predecessor, the cipher bureau or the "black chamber," managed to spy on allies and on Japan during talks on a naval disarmament treaty.