“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 N.S.A.’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 — used to justify airport screenings, for instance, or drunken-driving checkpoints — 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.”