"They were having a discussion and a light bulb went on," the official said.
The problem, according to the officials, 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, the official said, it was impossible to collect the targeted materials without also sweeping up data from innocent domestic U.S. users.
The officials did not explain why they did not prepare for that possibility when the surveillance program was created and why they discovered it only after the program was well under way.
Officials said that when they realized they had an American communication, the communication was destroyed. But it was not clear how they determined to whom an email belonged and whether any NSA analyst had actually read the content of the email. The officials said the bulk of the information was never accessed or analyzed.
As soon as the extent of the problem became clear, the officials said, the Obama administration provided classified briefings to both Senate and House intelligence committees within days. At the same time, officials also informed the FISA court, which later issued the three 2011 rulings released Wednesday — with redactions — as part of the government's latest disclosure of documents.
The officials briefed reporters on condition of anonymity because they were not authorized to describe the program publicly.
The documents were declassified to help the Obama administration explain some of the most recent disclosures made by The Washington Post after it published classified documents provided by former NSA systems analyst Edward Snowden.
One of the intelligence officials briefing reporters said the newly declassified documents should help explain "the reasons why people shouldn't go into a panic over articles they read in the press."