Once again from the U.K. Guardian:

The National Security Agency has a secret backdoor into its vast databases under a legal authority enabling it to search for US citizens'email and phone calls without a warrant, according to a top-secret document passed to the Guardian by Edward Snowden.
The previously undisclosed rule change allows NSA operatives to hunt for individual Americans' communications using their name or other identifying information. Senator Ron Wyden told the Guardian that the law provides the NSA with a loophole potentially allowing "warrantless searches for the phone calls or emails of law-abiding Americans".
The authority, approved in 2011, appears to contrast with repeated assurances from Barack Obama and senior intelligence officials to both Congress and the American public that the privacy of US citizens is protected from the NSA's dragnet surveillance programs.
The intelligence data is being gathered under Section 702 of the of theFisa Amendments Act (FAA), which gives the NSA authority to target without warrant the communications of foreign targets, who must be non-US citizens and outside the US at the point of collection.
The communications of Americans in direct contact with foreign targets can also be collected without a warrant, and the intelligence agencies acknowledge that purely domestic communications can also be inadvertently swept into its databases. That process is known as "incidental collection" in surveillance parlance.
But this is the first evidence that the NSA has permission to search those databases for specific US individuals' communications....
"Once Americans' communications are collected, a gap in the law that I call the 'back-door searches loophole' allows the government to potentially go through these communications and conduct warrantless searches for the phone calls or emails of law-abiding Americans."
Wyden, along with his intelligence committee colleague Mark Udall, have attempted repeatedly to warn publicly about the ability of the intelligence community to look at the communications of US citizens, but are limited by their obligation not to reveal highly classified information.
But in a letter they recently wrote to the NSA director, General Keith Alexander, the two senators warned that a fact sheet released by the NSA in the wake of the initial Prism revelations to reassure the American public about domestic surveillance was misleading...
Secret minimization procedures dating from 2009, published in June by the Guardian, revealed that the NSA could make use of any "inadvertently acquired" information on US persons under a defined range of circumstances, including if they held usable intelligence, information on criminal activity, threat of harm to people or property, are encrypted or are believed to contain any information relevant to cybersecurity....
Assurances from Obama and senior administration officials to the American public about the privacy of their communications have relied on the strict definition of what constitutes "targeting" while making no mention of the permission to search for US data within material that has already been collected.