Officials say the collection was inadvertent: The problem was that the top secret Internet-sweeping operation, which was targeting metadata contained in the emails of foreign users, was also amassing thousands of emails that were bundled up with the targeted materials. Because many web mail services use such bundled transmissions, it was impossible to collect the targeted materials without also sweeping up data from innocent domestic U.S. users.
The opinions, which were issued in 2011 and 2012, led the Foreign Intelligence Surveillance Court to order the NSA to find ways to limit what it collects and how long it keeps for.
The data -- gathered over three years -- was a small fraction of the 250 million communications collected by the NSA each year, according to the opinions.
White House spokesman Josh Earnest said the White House still contends there is no domestic surveillance program. He said the program is specifically to gather foreign intelligence, adding that the fact that the extent of incidental American surveillance has been documented is proof positive that accountability measures are working properly.
When they realized they had an American communication, the communication was destroyed, officials said, adding that the bulk of the information was never accessed or analyzed.
Officials said as soon as the extent of the problem became clear, the Obama administration provided classified briefings to both Senate and House intelligence committees within days. At the same time, officials also informed the secret Foreign Intelligence Surveillance Court, which later issued the three 2011 rulings released Wednesday - with redactions - as part of the government's latest disclosure of documents.
The declassified documents will be posted later in the day on Tumblr. An official said the documents should help explain " the reasons why people shouldn't go into a panic over articles they read in the press."
The Associated Press contributed to this report.