WASHINGTON -- In more than a dozen classified rulings, the nation's surveillance court has created a secret body of law giving the National Security Agency the power to amass vast collections of data on Americans while pursuing not only terrorism suspects, but also people possibly involved in nuclear proliferation, espionage and cyberattacks, officials say.
The rulings, some nearly 100 pages long, reveal that the court has taken on a much more expansive role by regularly assessing broad constitutional questions and establishing important judicial precedents, with almost no public scrutiny, according to current and former officials familiar with the court's classified decisions.
The 11-member Foreign Intelligence Surveillance Court, known as the FISA court, was once mostly focused on approving case-by-case wiretapping orders. But since major changes in legislation and greater judicial oversight of intelligence operations were instituted six years ago, it has quietly become almost a parallel Supreme Court, serving as the ultimate arbiter on surveillance issues and delivering opinions that will most likely shape intelligence practices for years to come, the officials said.
Last month, a former NSA contractor, Edward Snowden, leaked one classified order from the FISA court, which authorized the collection of all phone-tracing data from Verizon business customers. But the court's still-secret decisions go far beyond any single surveillance order, the officials said.
"We've seen a growing body of law from the court," a former intelligence official said. "What you have is a common law that develops where the court is issuing orders involving particular types of surveillance, particular types of targets."
In one of the court's most important decisions, the judges have expanded the use in terrorism cases of a legal principle known as the "special needs" doctrine and carved out an exception to the Fourth Amendment's requirement of a warrant for searches and seizures, the officials said.
The special needs doctrine was originally established in 1989 by the Supreme Court in a ruling allowing the drug testing of railway workers, finding that a minimal intrusion on privacy was justified by the government's need to combat an overriding public danger. Applying that concept more broadly, the FISA judges have ruled that the NSA's collection and examination of Americans' communications data to track possible terrorists does not run afoul of the Fourth Amendment, the officials said.
That legal interpretation is significant, several outside legal experts said, because it uses a relatively narrow area of the law and applies it much more broadly, in secret, to the wholesale collection of communications in pursuit of terrorism suspects.
"It seems like a legal stretch," William C. Banks, a national security law expert at Syracuse University, said in response to a description of the decision. "It's another way of tilting the scales toward the government in its access to all this data."
While President Barack Obama and his intelligence advisers have spoken of the surveillance programs leaked by Mr. Snowden mainly in terms of combating terrorism, the court has also interpreted the law in ways that extend into other national security concerns. In one recent case, for instance, intelligence officials were able to access an email attachment sent within the United States because they said they were worried that the email contained a schematic drawing or a diagram possibly connected to Iran's nuclear program.
In the past, that probably would have required a court warrant, because the suspicious email involved American communications. In this case, however, a little-noticed provision in a 2008 law, expanding the definition of "foreign intelligence" to include "weapons of mass destruction," was used to justify access to the message.
The court's use of that language has allowed intelligence officials to get wider access to data and communications that they believe may be linked to nuclear proliferation, the officials said. They added that other secret findings had eased access to data on espionage, cyberattacks and other possible threats connected to foreign intelligence.
"The definition of 'foreign intelligence' is very broad," another former intelligence official said in an interview.
The official, like a half-dozen other current and former national security officials, discussed the court's rulings and the general trends they have established on the condition of anonymity because they are classified. Judges on the FISA court would not comment on the scope and volume of their decisions.
Unlike the Supreme Court, the FISA court hears from only one side in the case -- the government -- and its findings are almost never made public. A Court of Review is empaneled to hear appeals, but that is known to have happened only a handful of times in the court's history, and no case has ever been taken to the Supreme Court. In fact, it is not clear in all circumstances whether Internet and phone companies that are turning over the reams of data even have the right to appear before the FISA court.
Created by Congress in 1978 as a check against wiretapping abuses by the government, the court meets in a secure, nondescript room in the federal courthouse in Washington. All of the current 11 judges, who serve seven-year terms, were appointed to the special court by Chief Justice John G. Roberts Jr., and 10 of them were nominated to the bench by Republican presidents. Most are from districts outside the capital and come in rotating shifts to hear surveillance applications; a single judge signs most surveillance orders, which totaled nearly 1,800 last year. None of the requests from the intelligence agencies was denied, according to the court.
The officials said that one central concept connects a number of the court's opinions. The judges have concluded that the mere collection of enormous volumes of "metadata" -- facts like the time of phone calls and the numbers dialed, but not the content of conversations -- does not violate the Fourth Amendment, as long as the government establishes a valid reason under national security regulations before taking the next step of actually examining the contents of an American's communications.