Washington - The National Security Agency appears to be collecting the telephone records of millions of American customers of Verizon, one of the nation's largest phone companies, under a top-secret court order issued in April.

The order appears to require Verizon to provide the NSA with daily information on all telephone calls by its customers within the United States and from foreign locations into the United States.

The order, which was signed by a judge from the secret court that oversees domestic surveillance, was first reported on the website of The Guardian newspaper. The website reproduced a copy of the order, which two former U.S. officials told The Washington Post appears to be authentic.

If the document is accurate, it would represent the broadest surveillance order known ever to have been issued. It also would confirm long-standing suspicions of civil liberties advocates about the sweeping nature of U.S. surveillance through commercial carriers under laws passed after the Sept. 11, 2001, terrorist attacks.

The order falls under Section 215 of the Patriot Act, which authorizes the government to make broad demands on telephone carriers for information about calls. In this case, the order requires Verizon to provide "ongoing, daily" information about "all call detail records ... created by Verizon for communications between the United States and abroad; or wholly within the United States, including local telephone calls."

The White House and the FBI, which apparently sought the order, refused to comment. A spokesman for Verizon also declined to comment.

But civil liberties groups were quick to criticize the sweeping nature of the order.

"This is a truly stunning revelation," said Elizabeth Goitein, co-director of the Liberty and National Security Program at the Brennan Center for Justice. "This suggests that the government has been compiling a comprehensive record of Americans' associations and possibly even their whereabouts."

The Center for Constitutional Rights, which has sued the government over its surveillance practices, said in a statement Wednesday night that the order "requires no level of suspicion and applies to all Verizon subscribers anywhere in the U.S. It also contains a gag order prohibiting Verizon from disclosing information about the order to anyone other than their counsel."

The order seems to confirm fears expressed by Democratic Sens. Ron Wyden of Oregon and Mark Udall of Colorado. In a letter to Attorney General Eric Holder last year, they said, "We believe most Americans would be stunned to learn the details of ... these secret court opinions. As we see it, there is now a significant gap between what most Americans think the law allows and what the government secretly claims the law allows."

Government officials have defended the broad surveillance powers, saying that the information has been vital in uncovering and disrupting terrorist plots.

The court order, good for three months, requires Verizon to hand over to the NSA, the world's largest spy agency, comprehensive communications routing information, including but not limited to numbers dialed and received, length of call, and customers' name and address or financial information. The order does not tell Verizon to provide any call content information.

The "business records" order, issued by the Foreign Intelligence Surveillance Court, does not require a showing of probable cause to believe that the targets are agents of a foreign power. Rather, all that is required is a showing that "there are reasonable grounds to believe" that the tangible things sought are "relevant to an authorized investigation ... to obtain foreign intelligence information ... or to protect against international terrorism or clandestine intelligence activities."

"Tell me how all of the metadata of Americans' domestic conversations can be relevant to an authorized foreign intelligence or international terrorism investigation," said Goitein, who was counsel to then-Sen. Russell Feingold, D-Wis., a Senate Judiciary Committee member who left Congress in January 2011.

Feingold raised concerns about the government's secret interpretation of Section 215, a concern picked up by Wyden and Udall in recent years. These lawmakers have pressed the Justice Department and the surveillance court to release redacted versions of their opinions so that Americans can understand the full extent of the government's surveillance authorities.

The order does not specify whether location data, such as cell tower or GPS coordinates, also are being turned over. But even without such data, the NSA is still gathering massive amounts of information that can give a detailed picture of people's networks of associates, with whom they are communicating, when and for how long.

"It's the kind of networking information that can be very important in legitimate terrorism investigations but that the government has no business knowing about ordinary law-abiding Americans," Goitein said.

Privacy advocates said they believed that the NSA must be collecting such information from all the large carriers and that this practice has been going on for years.