Top secret documents leaked to the Washington Post show that the US National Security Agency violated privacy rules thousands of times since 2008.
The findings were provided to the Post and the Guardian by former NSA contractor Edward Snowden, who is currently living in Russia under temporary asylum.
The audit of the NSA details widespread unauthorized surveillance of Americans' communications, from telephone calls to emails. The breaches ranged from program errors to serious infractions.
The most serious violation was a diversion of massive amounts of data through fiber-optic cables in the US that included a large amount of communications by Americans.
The diversion was judged by the Foreign Intelligence Surveillance Court (FISA) as unconstitutional, but the court opinion has been hidden and the Obama administration has fought the a Freedom of Information Act request to see it.
The new findings also show unauthorized surveillance of foreign intelligence targets that entered the United States.
The Washington Post spoke to officials from the NSA before publishing the report Thursday.
“We’re a human-run agency operating in a complex environment with a number of different regulatory regimes, so at times we find ourselves on the wrong side of the line,” a senior NSA official told the Post in an interview, speaking with White House permission on the condition of anonymity.
The NSA audit dated May 2012 shows 2,776 violations in the last 12 months, with most considered unintended.
For example, interceptions of a large number of calls out of Washington occurred after computers confused the Egyptian country code 20 for the DC area code 202.
A troubling detail is that the violations increased from 546 in the second quarter of 2011 to 865 in the first quarter of 2012. It is unclear if they have continued to increase.
The audit only counts the errors by the offices in the Washington area and by the NSA headquarters in Maryland. The report shows that the NSA removed details of its programs when sending documents to the Justice Department and the Office of the Director of National Intelligence.
The Foreign Intelligence Surveillance Court, which has been criticized for its secrecy and kowtowing to US security authorities, was apparently kept in the dark about new collection methods.
The Post report seems to suggest that the NSA trained analysts to cover up incidents that would be reported to the FISA court [or FAA] and to purposefully obscure information.
"In one required tutorial, NSA collectors and analysts are taught to fill out oversight forms without giving 'extraneous information' to 'our FAA overseers,'" the Post wrote.
The Washington Post pointed out that the documents show a level of detail that is not even available to US lawmakers nor the FISA court, which oversees the programs.
Though some of the audits are made available to members of Congress, they are only allowed to see them in a secure room and are not allowed to take notes. Many Congress members, the Post pointed out, do not have advisors with security clearance and staff who could adequately explain the meaning of the infractions.
The documents available also don't reveal how many Americans are actually affected by the violations.
The new revelations are likely to increase pressure on the Obama administration, which has steadfastly defended the surveillance programs.