The National Security Agency had access to the browsing, email and chat history of millions of people worldwide, according to documents provided to The Guardian.
Dubbed XKeyscore, the program allowed analysts to dig through enormous databases of personal computer data without authorization or permission from end users.
The information was provided to Glenn Greenwald and The Guardian by Edward Snowden, who remains on the run in Russia from American authorities.
The revelations seem to back up his claim from early June that NSA analysts have the ability to tap into vast expanses of personal data.
"I could wiretap anyone...if I had a personal email," Snowden told Greenwald.
In a presentation posted to The Guardian's website, officials said that XKeyscore was the agency's "widest reaching" system with the purpose of collecting "digital intelligence." The documents indicate that the program covered "nearly everything" a typical user does online.
Snowden admitted on June 9 that he was the one behind leaking secret NSA documents to Greenwald. At the time, he was hiding in Hong Kong, but has since spend a great amount of time in Russia seeking asylum.
A United States official denied Snowden's claims, The Guardians says.
His previous revelations indicated that the NSA had gathered hundreds of millions of phone records, while another allowed the government to access data from a number of major Internet service providers to track overseas usage and activity.
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