It is considered "inadvertently" collected if the target actually turns out to be an American, according to program rules and people familiar with them. The extent of incidental and inadvertent collection has not been disclosed, leading some lawmakers to demand disclosure of estimates of how many Americans' communications have been gathered. No senior intelligence officials have answered that question publicly.
Using software that scans traffic and "sniffs out" the targeted email address, the company can pull out email traffic automatically to turn over to the government, according to several former government officials and industry experts.
It is unclear how effective that approach is compared with collecting from a "downstream" tech company such as Google or Facebook, but the existence of separate programs collecting data from both technology companies and telecommunications systems underscores the reach of government intelligence agencies.
"People need to realise that there are many ways for the government to get vast amounts of email," said Chris Soghoian, a technology expert with the American Civil Liberties Union.
The drive for new intelligence sources after the September 11, 2001, attacks relied on a key insight: American companies controlled most of the internet's essential pipes, giving ample opportunities to tap the torrents of data flowing by. Even terrorists bent on destruction of the United States, it turned out, talked to each other on web-based programs such as Microsoft's Hotmail.
Yet even data not handled by US-based companies generally flowed across parts of the American telecommunications infrastructure. Most important were the fibre-optic cables that largely have replaced the copper telephone wires and the satellite and microwave transmissions that, in an earlier era, were the most important targets for government surveillance.
Fibre-optic cables, many of which lie along the ocean floor, provide higher-quality transmission and greater capacity than earlier technology, with the latest able to carry thousands of gigabits per second.
The world's hundreds of undersea cables now carry 99 per cent of all intercontinental data, a category that includes most international phone calls as well, says TeleGeography, a global research firm.
The fibre-optic networks have become a rich source of data for intelligence agencies.The Guardian newspaper reported last month that the Government Communications Headquarters, the British equivalent of the NSA, taps and stores data flowing through the fiber-optic cables touching that nation, a major transit point for data between Europe and the Americas.
That program, code-named Tempora, shares data with the NSA, the newspaper said.
Tapping undersea transmission cables had been a key US surveillance tactic for decades, dating back to the era when copper lines carrying sensitive telephone communications could be accessed by listening devices divers could place on the outside of a cable's housing, said naval historian Norman Polmar, author of Spy Book: The Encyclopedia of Espionage.
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