The telephone records collection program was authorized by the Foreign Surveillance Intelligence Court under what Clapper called "stringent condition." It's reviewed every 90 days, he said. Feinstein said the order published in The Guardian was the approval of one such 90-day review.
The court is special judicial office set up as part of the 1978 Foreign Intelligence Surveillance Act. The court operates in secrecy, reviewing requests by intelligence agencies to conduct electronic surveillance and other activities as part of espionage, terrorism and national security investigations.
Is it legal?
The FISA Court, Clapper and other administration officials obviously think so. But many privacy advocates and some lawmakers believe this sort of broad data collection goes too far.
"While I cannot corroborate the details of this particular report, this sort of widescale surveillance should concern all of us and is the kind of government overreach I've said Americans would find shocking," said Sen. Mark Udall, D-Colorado, who serves on the Senate Intelligence Committee.
In a letter last year to Attorney General Eric Holder, Udall and Sen. Ron Wyden, D-Oregon, complained about secret interpretations of the Patriot Act, details of which are classified but which the senators said "most Americans would be stunned to learn the details of."
"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," the senators wrote. "This is a problem, because it is impossible to have an informed public debate about what the law should say when the public doesn't know what its government thinks the law says."
Has this happened before?
In 2006, it was revealed that the NSA was secretly collecting telephone records as part of an effort to root out potential terror plots.
At that time, Verizon denied reports that it was providing the NSA with data from customers' domestic calls. The company said that while it is committed to helping the government protect against terrorist attacks, "we will always make sure that any assistance is authorized by law and that our customers' privacy is safeguarded."
The Electronic Frontier Foundation is also suing the NSA over claims that it was working with AT&T and possibly other telecommunications companies to suck up enormous amounts of Internet traffic through secure NSA-controlled rooms attached to network stations.
Other programs, some going back decades, have stoked similar concerns. In fact, abuses by intelligence services led to the passage of the Foreign Intelligence Surveillance Act in an effort to rein in domestic surveillance practices.