President Barack Obama's national security has team acknowledged for the first time that it has the ability to read the phone records of millions of Americans while looking for just one terrorism suspect.
Appearing before the Senate judiciary committee, John Inglis, the NSA's deputy director, conceded that his agents can track the telephone activities of millions of Americans while searching for one terrorism suspect, but said that agents "try to be judicious" in their searches.
The Obama administration has previously stated that such records are rarely searched and, when they are, officials target only suspected foreign terrorists.
The searches described to the judiciary committee hinge on the "chain" analysis of information gathered on telephone communications. When the NSA identifies a suspect, it can look not just at their phone records, but also the records of everyone they call, everyone who calls those people and everyone who calls those people.
If the average person called 40 people, the analysis would allow the government to mine the records of 2.5 million Americans when investigating one suspect. The NSA conducted 300 such searches last year.
The Democratic senator, Dick Durbin, said: "So what has been described as a discrete programme, to go after people who would cause us harm, when you look at the reach of this programme, it envelopes a substantial number of Americans."
"We are open to re-evaluating this programme in ways that can perhaps provide greater confidence and public trust that this is in fact a programme that achieves both privacy protections and national security,'' Robert Litt, counsel to the director of National Intelligence, told the judiciary committee.
The admission came on the same day that fresh information on NSA spying was revealed by the Guardian newspaper.
Citing documents from whistle-blower Edward Snowden, the newspaper published NSA training material for the "XKeyscore" programme, which it described as the NSA's widest-reaching system that covers "nearly everything a typical user does on the Internet".
Intelligence analysts can conduct surveillance through XKeyscore by filling in an on-screen form giving only a "broad justification" for the search, and no review by a court or NSA staff, the newspaper said.
The search could then trawl internet search histories, emails and other personal information of those targeted.
In response to the report, the agency said: "The implication that NSA's collection is arbitrary and unconstrained is false", and added that XKeyscore was part of "NSA's lawful foreign signals intelligence collection system".
Through a prism
Meanwhile, the director of national intelligence released three declassified documents on Wednesday in the "interest of increased transparency". They explained the bulk collection of phone data - one of the secret programmes revealed by Snowden.
Much of what is in the newly declassified documents has already been divulged in public hearings by intelligence officials. The released documents included 2009 and 2011 reports on the NSA's "Bulk Collection Programme", carried out under the US Patriot Act, the anti-terrorism legislation passed shortly after the September 11 attacks.
They also included an April 2013 order from the Foreign Intelligence Surveillance Court, which directed communications company Verizon to hand over data from millions of Americans' telephone calls.