The government is secretly collecting the telephone records of millions of U.S. customers of Verizon under a top-secret court order, according to the chairwoman of the Senate Intelligence Committee.

The Obama administration is defending the National Security Agency's need to collect such records, but critics are calling it a huge over-reach. Sen. Dianne Feinstein, D-Calif., told reporters Thursday that the court order for telephone records, first disclosed by The Guardian newspaper in Britain, was a three-month renewal of an ongoing practice.

The records have been collected for some seven years, according to Sen. Harry Reid, D-Nev. "I think people want the homeland kept safe to the extent we can," Feinstein said at a Capitol Hill news conference. "We want to protect these privacy rights. That's why this is carefully done in federal court with federal judges who sit 24/7 who review these requests.''

And the chairman of the House Intelligence Committee, Republican Rep. Mike Rogers of Michigan, said the NSA search of telephone records had thwarted an attempted terrorist attack in the United States in the past few years.

He said it was a "significant case" but declined to provide further details.

White House spokesman Josh Earnest said that he couldn't provide classified details but that the court order in question is a critical tool for learning when terrorists or suspected terrorists might be engaging in dangerous activities. He said there are strict legal rules on how such a program is conducted and that congressional leaders are briefed. Vermont Sen. Bernie Sanders, an independent whose comments were echoed by several members of both parties, said: ``To simply say in a blanket way that millions and millions of Americans are going to have their phone records checked by the U.S. government is to my mind indefensible and unacceptable.''

The disclosure raised a number of questions: What is the government looking for? Are other big telephone companies under similar orders to turn over information?

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How is the information used and how long are the records kept?

The sweeping roundup of U.S. phone records has been going on for years and was a key part of the Bush administration's warrantless surveillance program, according to a U.S. official. Attorney General Eric Holder sidestepped questions about the issue during an appearance before a Senate subcommittee, offering instead to discuss it at a classified session that several senators said they would arrange. The order was granted by the secret Foreign Intelligence Surveillance Court on April 25 and is good until July 19, the Guardian reported.

MORE: How does NSA phone-record program work?

It requires Verizon, one of the nation's largest telecommunications companies, on an "ongoing, daily basis,'' to give the NSA information on all landline and mobile telephone calls of Verizon Business in its systems, both within the U.S. and between the U.S. and other countries. The document shows for the first time that under the Obama administration, the communication records of millions of U.S. citizens are being collected indiscriminately and in bulk, regardless of whether the people are suspected of any wrongdoing. A former U.S. intelligence official who is familiar with the NSA program said that records from all U.S. phone companies would be seized by the government under the warrants, and that they would include business and residential numbers.

Reaction to the revelation both pro and con reflected the vigorous debate in Washington over how best to balance the sometimes-competing goals of protecting the nation from terror attacks while safeguarding the privacy and civil rights of Americans. President Barack Obama, in a recent national security address, said the nation is at a crossroads as it determines how to remain vigilant yet move beyond a post-9/11 mindset focused on global antiterrorism. Former Vice President Al Gore tweeted that privacy was essential in the digital era.

"Is it just me, or is secret blanket surveillance obscenely outrageous?" wrote Gore, the Democrat who lost the 2000 presidential election to George W. Bush. But Sen. Lindsey Graham, R-S.C., said he had no problem with the court order and the practice, declaring, ``If we don't do it, we're crazy.''

"If you're not getting a call from a terrorist organization, you've got nothing to worry about," he said.

Arizona Sen. John McCain, who ran against Obama for president in 2008, said that if the records sweep was designed to track "people in the United States who are communicating with members of jihadist terrorist organizations," that might not be a problem.

"But if it was something where we just blanket started finding out who everybody called and under what circumstances, then I think it deserves congressional hearings.'' Senate Democratic leader Reid played down the significance of the revelation.

"Right now I think that everyone should just calm down and understand that this isn't anything that's brand new," he said."This is a program that's been in effect for seven years, as I recall. It's a program that has worked to prevent not all terrorism but certainly the vast, vast majority. Now is the program perfect? Of course not."

The disclosure of the records sweep was just the latest controversy to hit the Obama administration.

The president also is facing questions over the Internal Revenue Service's improper targeting of conservative groups, the seizure of journalist phone records in an investigation into who leaked information to the media, and the administration's handling of the terrorist attack in Libya that left four Americans dead. At the very least, the controversies threaten to distract the White House at a pivotal time, when the president wants to tackle big issues like immigration reform and taxes. At most, the controversies collectively could erode the American people's trust in him, threatening both to derail his second term agenda and sully his presidential legacy.

The court order did not authorize snooping into the content of phone calls. But with millions of phone records in hand, the NSA's computers can analyze them for patterns, spot unusual behavior and identify what are known in intelligence circles as "communities of interest" networks of people in contact with targets or suspicious phone numbers overseas. Once the government has zeroed in on numbers it believes are tied to terrorism or foreign governments, 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.

The court document related to Verizon offers a glimpse into the larger NSA effort.

Under the law, the government would need to demand records from each phone company individually. While subpoenas for other phone companies have not been made public, for the data-mining program described by government officials to work, the government would need records for all providers.

"There is no indication that this order to Verizon was unique or novel. It is very likely that business records orders like this exist for every major American telecommunication company, meaning that if you make calls in the United States the NSA has those records. And this has been going on for at least 7 years, and probably longer," wrote Cindy Cohn, general counsel of the nonprofit digital rights group Electronic Frontier Foundation, and staff attorney Mark Rumold, in a blog post.

Jim Harper, a communications and privacy expert at the libertarian-leaning Cato Institute, questioned the effectiveness of using so-called pattern analyses to intercept terrorism. He said that kind of analysis _ finding trends in transactional data collected by Verizon would produce many false positives and give the government access to intricate data about people's calling habits.

"This is not just entertainment or a sideshow. This is a record of who you called every day this month," he said, urging Congress to require the government to provide "a full explanation" of how this data turns up terrorism plots. Under Bush, the National Security Agency built a highly classified wiretapping program to monitor emails and phone calls worldwide.

The full details of that program remain unknown, but one aspect was to monitor massive numbers of incoming and outgoing U.S. calls to look for suspicious patterns, said an official familiar with the program.

That official spoke on condition of anonymity because he was not authorized to discuss it publicly.

After The New York Times revealed the existence of that wiretapping program, the roundup continued under authority granted in the USA Patriot Act, the official said. The official did not know if the program was continuous or whether it stopped and restarted at times.

The official had not seen the court order released by the Guardian newspaper but said it was consistent with similar authorizations the Justice Department has received. Verizon spokesman Ed McFadden said Wednesday the company had no comment.

The NSA had no immediate comment. The agency is sensitive to perceptions that it might be spying on Americans. In a brochure it distributes, which includes a DVD for reporters to view video that it provides for public relations purposes, it pledges that the agency "is unwavering in its respect for U.S. laws and Americans' civil liberties and its commitment to accountability,'' and says, "Earning the American public's trust is paramount.'' Verizon Communications Inc. listed 121 million customers in its first-quarter earnings report this April 98.9 million wireless customers, 11.7 million residential phone lines and about 10 million commercial lines.

The court order didn't specify which customers' records were being tracked. Under the terms of the order, the phone numbers of both parties on a call are handed over, as are call time and duration, and unique identifiers.

The contents of the conversation itself are not covered, The Guardian said. A senior administration official, defending the collection of phone records by the government, said, "On its face, the order reprinted in the article does not allow the government to listen in on anyone's telephone calls.''

The official spoke on condition of anonymity because the official was not authorized to discuss the matter publicly.

Interviewed separately, the former intelligence official described a system in which the database needed to be kept current so that if intelligence agencies obtained a phone number from a terror suspect overseas, it could immediately be matched against the records on file.

Because it is not easy or quick to obtain the records from phone companies, the Obama administration needed to renew the Bush-era program on an ongoing basis to keep it updated, the former official said. It's not clear how long the records are kept, or if they are destroyed. The former official said that since terror suspects frequently change phone numbers to cover their tracks, there is little need for older records.

Congressional intelligence agencies were briefed extensively on the program, and received support from both Republicans and Democrats to continue it, the former official said.

"Some were nervous about it, but there was never any attempt to stop the program,'' he said. He described it as a White House-led process. The broad, unlimited nature of the records being handed over to the NSA is unusual.

FISA court orders typically direct the production of records pertaining to a specific named target suspected of being an agent of a terrorist group or foreign state, or a finite set of individually named targets.

NSA warrantless wiretapping during the George W. Bush administration after the 9/11 attacks was very controversial. The FISA court order, signed by Judge Roger Vinson, compelled Verizon to provide the NSA with electronic copies of "all call detail records or telephony metadata created by Verizon for communications between the United States and abroad'' or "wholly within the United States, including local telephone calls,'' The Guardian said.

The law on which the order explicitly relies is the "business records" provision of the USA Patriot Act.