America's ultra-secret National Security Agency reluctantly finds itself in the headlines amid a wave of disclosures from ex-intelligence contractor Edward Snowden, who has exposed the service's vast electronic spying operation.
France and Mexico both demanded explanations Monday after the latest revelations from Snowden alleged the NSA secretly monitored tens of millions of phone communications in France and hacked into former Mexican president Felipe Calderon's email account.
Hollywood directors and novelists have made the CIA famous for its undercover agents in the field, but in the digital era, the high-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.
The agency uses super computers, linguists and code-breaking mathematicians to oversee what experts say is the world's most powerful digital espionage organization, scooping up phone conversations and email traffic 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 a string of 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 new Cyber Command for digital warfare, and the service plays a crucial role in securing computer networks against a cyber attack.
The NSA's budget remains classified but it is believed to be the largest in the intelligence community. Funding doubled since the attacks of Sept. 11, 2011, according to the book "Top Secret America" by journalists Dana Priest and William Arkin.
The agency, with its sprawling headquarters at 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 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 the NSA intercepts more than a billion emails, phone calls and other types of communications, according to "Top Secret America."
To retain the massive amount of data, the agency is constructing a vast storage center in the Utah desert at a cost of US$2 billion, which will serve as 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 the 1920s, code-breakers at NSA's predecessor, the cipher bureau or the "black chamber," spied on allies and on Japan during talks on a naval disarmament treaty.