At issue is a court order, first disclosed on Wednesday by The Guardian newspaper in Britain, that requires the communications company Verizon to turn over on an "ongoing, daily basis" the records of all landline and mobile telephone calls of its customers, both within the US and between the US and other countries.
Intelligence experts said the government, though not listening in on calls, would be looking for patterns that could lead to terrorists — and that there was every reason to believe similar orders were in place for other phone companies.
Some critics in Congress, as well as civil liberties advocates, declared that the sweeping nature of the National Security Agency programme represented an unwarranted intrusion into Americans' private lives. But a number of lawmakers, including some Republicans who normally jump at the chance to criticise the Obama administration, praised the programme's effectiveness. Leaders of the House Intelligence Committee said the programme had helped thwart at least one attempted terrorist attack in the United States.
Separately, The Washington Post and The Guardian reported on Thursday the existence of another programme used by the NSA and FBI that scours the nation's main internet companies, extracting audio, video, photographs, e-mails, documents and connection logs to help analysts track a person's movements and contacts. It was not clear whether the programme, called Prism, targets known suspects or broadly collects data from other Americans.
The companies include Microsoft, Yahoo, Google, Facebook, PalTalk, AOL, Skype, YouTube and Apple. The Post said PalTalk has had numerous posts about the Arab Spring and the Syrian civil war. It also said Dropbox would soon be included.
Google, Facebook and Yahoo said in statements that they do not provide the government with direct access to their records. In a statement, Google said it discloses user data to the government in accordance with the law and reviews all such requests carefully. "From time to time, people allege that we have created a government 'back door' into our systems, but Google does not have a 'back door' for the government to access private user data," the statement said.
The leaks about the programmes brought a sharp response from James Clapper, the director of national intelligence. In an unusual statement late on Thursday, Clapper called disclosure of the internet surveillance programme "reprehensible" and said the leak about the phone record collecting could cause long-lasting and irreversible harm to the nation's ability to respond to threats.
Clapper said news reports about the programmes contained inaccuracies and omitted key information. He declassified some details about the authority used in the phone records programme because he said Americans must know the programme's limits. Those details included that a special national security court reviews the programme every 90 days and that the court prohibits the government from indiscriminately sifting through phone data. Queries are only allowed when facts support reasonable suspicion, Clapper said.
One outraged senator, Ron Wyden, said of the phone-records collecting: "When law-abiding Americans make phone calls, who they call, when they call and where they call is private information. As a result of the discussion that came to light today, now we're going to have a real debate."
But Republican Lindsey Graham said Americans have no cause for concern. "If you're not getting a call from a terrorist organisation, you've got nothing to worry about," he said.
A senior administration official pointed out that the collection of communication cited in the Washington Post and Guardian articles involves "extensive procedures, specifically approved by the court, to ensure that only non-US persons outside the US are targeted, and that minimise the acquisition, retention and dissemination of incidentally acquired information about US persons". The official, who was not authorised to discuss the matter publicly and requested anonymity, added that Congress had recently re-authorised the programme.
Senate Intelligence Committee Chairperson Dianne Feinstein said the order was a three-month renewal of an ongoing practice that is supervised by federal judges who balance efforts to protect the country from terror attacks against the need to safeguard Americans' privacy. The surveillance powers are granted under the post-9/11 Patriot Act, which was renewed in 2006 and again in 2011.
The disclosure offered a public glimpse into a programme whose breadth is not widely understood. Senator Mark Udall, who serves on the Intelligence Committee, said it was the type of surveillance that "I have long said would shock the public if they knew about it."
The government has hardly been forthcoming.
Wyden released a video of himself pressing Director of National Intelligence James Clapper on the matter during a Senate hearing in March.
"Does the NSA collect any type of data at all on millions or hundreds of millions of Americans?" Wyden asked.
"No, sir," Clapper answered.
'Worst fears confirmed'
"It does not?" Wyden pressed.
Clapper quickly softened his answer. "Not wittingly," he said. "There are cases where they could, inadvertently perhaps, collect — but not wittingly."
There was no immediate comment from Clapper's office on Thursday on that testimony.
"This confirms our worst fears," said Alexander Abdo, a staff attorney with the American Civil Liberties Union's National Security Project. "If the government can track who we call, the right to privacy has not just been compromised — it has been defeated."
Attorney General Eric Holder sidestepped questions about the issue during an appearance before a Senate subcommittee on Thursday, offering instead to discuss it at a classified session that several senators said they would arrange.
The Verizon order, granted by the secret Foreign Intelligence Surveillance Court on 25 April and good until 19 July, requires information on the phone numbers of both parties on a call, as well as call time and duration, and unique identifiers, according to The Guardian.
It does not authorise snooping into the content of phone calls. But with millions of phone records in hand, the NSA's computers can analyse them for patterns, spot unusual behaviour and identify networks of people in contact with targets or suspicious phone numbers overseas.
Once the government has zeroed in on numbers, it can go back to the court with a wiretap request. That allows the government to monitor the calls in real time, record them and store them indefinitely.
Jim Harper, a communications and privacy expert at the libertarian-leaning Cato Institute, said that kind of analysis would produce many false positives and give the government access to intricate data about people's calling habits.
Verizon Executive Vice President and General Counsel Randy Milch, in a blog post, said the company isn't allowed to comment on any such court order.
The NSA had no immediate comment.