WASHINGTON (AP) -- In the months and early years after 9/11, FBI agents began showing up at Microsoft Corp. more frequently than before, armed with court orders demanding information on customers.
Around the world, government spies and eavesdroppers were tracking the email and Internet addresses used by suspected terrorists. Often, those trails led to the world's largest software company and, at the time, largest email provider.
The agents wanted email archives, account information, practically everything, and quickly. Engineers compiled the data, sometimes by hand, and delivered it to the government.
Often there was no easy way to tell if the information belonged to foreigners or Americans. So much data was changing hands that one former Microsoft employee recalls that the engineers were anxious about whether the company should cooperate.
Inside Microsoft, some called it "Hoovering" -- not after the vacuum cleaner, but after J. Edgar Hoover, the first FBI director, who gathered dirt on countless Americans.
This frenetic, manual process was the forerunner to Prism, the recently revealed highly classified National Security Agency program that seizes records from Internet companies. As laws changed and technology improved, the government and industry moved toward a streamlined, electronic process, which required less time from the companies and provided the government data in a more standard format.
The revelation of Prism this month by the Washington Post and Guardian newspapers has touched off the latest round in a decade-long debate over what limits to impose on government eavesdropping, which the Obama administration says is essential to keep the nation safe.
But interviews with more than a dozen current and former government and technology officials and outside experts show that, while Prism has attracted the recent attention, the program actually is a relatively small part of a much more expansive and intrusive eavesdropping effort.
Americans who disapprove of the government reading their emails have more to worry about from a different and larger NSA effort that snatches data as it passes through the fiber optic cables that make up the Internet's backbone. That program, which has been known for years, copies Internet traffic as it enters and leaves the United States, then routes it to the NSA for analysis.
Whether by clever choice or coincidence, Prism appears to do what its name suggests. Like a triangular piece of glass, Prism takes large beams of data and helps the government find discrete, manageable strands of information.
The fact that it is productive is not surprising; documents show it is one of the major sources for what ends up in the president's daily briefing. Prism makes sense of the cacophony of the Internet's raw feed. It provides the government with names, addresses, conversation histories and entire archives of email inboxes.
Many of the people interviewed for this report insisted on anonymity because they were not authorized to publicly discuss a classified, continuing effort. But those interviews, along with public statements and the few public documents available, show there are two vital components to Prism's success.
The first is how the government works closely with the companies that keep people perpetually connected to each other and the world. That story line has attracted the most attention so far.
The second and far murkier one is how Prism fits into a larger U.S. wiretapping program in place for years.
Deep in the oceans, hundreds of cables carry much of the world's phone and Internet traffic. Since at least the early 1970s, the NSA has been tapping foreign cables. It doesn't need permission. That's its job.
But Internet data doesn't care about borders. Send an email from Pakistan to Afghanistan and it might pass through a mail server in the United States, the same computer that handles messages to and from Americans. The NSA is prohibited from spying on Americans or anyone inside the United States. That's the FBI's job and it requires a warrant.
Despite that prohibition, shortly after the Sept. 11 terrorist attacks, President George W. Bush secretly authorized the NSA to plug into the fiber optic cables that enter and leave the United States, knowing it would give the government unprecedented, warrantless access to Americans' private conversations.
Tapping into those cables allows the NSA access to monitor emails, telephone calls, video chats, websites, bank transactions and more. It takes powerful computers to decrypt, store and analyze all this information, but the information is all there, zipping by at the speed of light.
"You have to assume everything is being collected," said Bruce Schneier, who has been studying and writing about cryptography and computer security for two decades.