The National Security Agency, nicknamed "No Such Agency" because of its ultra-secrecy, is the government's eavesdropper-in-chief.
The agency is charged primarily with electronic spying around the globe. The NSA collects billions of pieces of foreign "signals intelligence" using its intercept systems. But as news reports in recent days have indicated, it also gathers information on the electronic and phone conversations of millions of ordinary Americans.
Regarded as the most secretive of the nation's intelligence agencies, the NSA is part of the military but answers to the director of national intelligence. Its major operations are housed at Maryland's Fort Meade Army base.
Estimates of the number of employees range from the official figure of about 35,000 to as high as 55,000. In addition to its main campus behind the walls of Fort Meade, the NSA will operate a new surveillance center in the Utah desert. The million-square-foot building, five times larger than the Capitol, will cost about $2 billion when it's finished, perhaps as early as the fall.
The center is designed to capture all forms of communication for the nation's intelligence agencies, ranging from email and cellphone calls to Internet searches and personal data. James Bamford, a best-selling author who has written extensively about the NSA in books with telling titles including "The Puzzle Palace" and "The Shadow Factory," has estimated the surveillance center could store data equal to 500 quintillion pages.
Protecting U.S. secrets and cracking the codes of its enemies are as old as the Revolutionary War. The NSA's origins are traced to military radio interceptions and code-breaking during World Wars I and II.
Since then, the agency's mission expanded dramatically, following advances in communications. The result is that the NSA has grown into the largest and most technologically sophisticated spying organization in the nation and possibly the world. And, in the view of some civil liberties experts, it is one of the most intrusive.
Today, the agency listens to millions of phone calls worldwide, analyzes the content and cracks codes, all essentially defensive activities. Little is known about the other side of the coin: the extent of the offensive work by the NSA, such as planting computer viruses or otherwise disrupting suspected terrorist communications.
Declassified documents show that since at least 1997, the NSA has been charged with developing ways to attack hostile computer networks. For example, the Stuxnet virus, developed to damage Iran's nuclear programs, was a collaboration between NSA scientists and technicians and their counterparts within Israel's espionage apparatus, according to U.S. officials.
The NSA's main mission remains collecting and analyzing electronic data. But since the Sept. 11, 2001, terror attacks, the agency has increasingly focused on protecting U.S. government computer networks.
The budget is classified. Matthew Aid, who wrote a 2009 history of the agency based largely on declassified documents, estimated the annual budget then at $8 billion. No one thinks it has dropped since.
Aid estimates NSA personnel could number as high as 55,000, including 30,000 employees in the United States and overseas for the agency and another 25,000 monitoring and intercepting phone calls, radio signals and radar for the military branches.
Gen. Keith B. Alexander took over as head of the NSA in 2005. More recently, he also became head of U.S. Cyber Command, which develops new types of warfare.
At the end of Alexander's first year at the NSA, The New York Times disclosed that the George W. Bush administration had authorized the agency to run a vast, warrantless domestic spying program. The extensive surveillance has continued under the Obama administration, although officials contend it operates within the parameters of the Patriot Act.
Two weeks ago at a conference in Washington, Alexander argued that the NSA has its hands full keeping tabs on potential terrorists and does not have the bandwidth to read the 420 billion emails generated by Americans daily. Some foreign governments are trying to do just that, he said.
"The great irony is we're the only ones not spying on the American people," the Reuters news agency quoted him as saying.