It also has triggered a push in Congress this week to pass temporary legislation that would protect parts of a controversial eavesdropping program launched by the Bush administration after the Sept. 11 attacks.
The administration and Democrats are at odds over how to address the issue, leading to concerns that it might not be resolved before Congress starts its August recess Monday.
This week, congressional leaders have alluded to the recent decision by the court, which was created in 1978 as part of the Foreign Intelligence Surveillance Act.
House Minority Leader John A. Boehner (R-Ohio) said in a television interview Tuesday evening: "There's been a ruling, over the last four or five months, that prohibits the ability of our intelligence services and our counterintelligence people from listening in to two terrorists in other parts of the world where the communication could come through the United States."
Senate Intelligence Committee Chairman John D. Rockefeller IV (D-W.Va.) said Wednesday that "recent technical developments" had convinced him that "we must take some immediate but interim step to improve collection of foreign intelligence in a manner that doesn't compromise civil liberties of U.S. citizens."
Neither Rockefeller nor Boehner would elaborate, but U.S. intelligence and congressional officials familiar with the matter said they were referring to the FISA court ruling.
Boehner's remarks suggest that the ruling imposed new restrictions on the National Security Agency's ability to intercept communications that are between people overseas but that "transit" U.S. data networks operated by Internet service providers and telecommunications companies.
But other officials said the ruling's reach was broader, affecting cases "where one end is foreign and you don't know where the other is" — meaning warrants would be required even when it was unclear whether communications were crossing the United States or involved a person in the United States.
One official said the issue centered on a ruling in which a FISA court judge rejected a government application for a "basket warrant" — a term that refers to court approval for surveillance activity encompassing multiple targets, rather than warrants issued on a case-by-case basis for surveillance of specific terrorism suspects.
"One FISA judge approved this, and then a second FISA judge didn't," the official said, speaking on condition of anonymity because the activities of the FISA court are classified.
The precise effect of the ruling is unclear, but a second official said that it "reduced the amount of intelligence we were collecting" on overseas terrorism suspects.
National Intelligence Director J. Michael McConnell has called attention to the issue in public testimony, telling a Senate committee May 1 that U.S. spy agencies are "actually missing a significant portion of what we should be getting" because of legal obstacles. But he has refused to identify the cause or nature of that intelligence gap.
The recent FISA court ruling was a blow to the Bush administration, which had bypassed the court when it launched the NSA program in 2001. The White House moved it back under the FISA court's supervision last year after Democrats won control of Congress and appeared poised to challenge the constitutionality of a program that monitored U.S. residents' communications without warrants.
The ruling comes at a time when U.S. intelligence agencies have warned that Al Qaeda has regrouped and is refocusing its energies on striking the U.S.
The issue has become the center of a fierce new debate on Capitol Hill over how to update the Foreign Intelligence Surveillance Act, which requires the government to get a special court's approval before monitoring communications of people in the U.S. Public records show that the court rejects few of the government's requests: In 2005, for example, it approved 2,072 applications and denied none; in 2006 it approved 2,176 and denied, in part, one.
This week, unable to agree on a broader overhaul, the Bush administration and congressional leaders have turned instead to passing temporary legislation designed to address concerns raised about the recent court ruling. Even so, they remain at odds over the FISA court's role.
Democrats have proposed a temporary fix that would give the FISA court new authority to grant court orders covering "certain aggregated foreign collection while protecting rights and privacy of U.S. persons."
But the Bush administration has pushed for broader language eliminating any requirement for a court order in cases where the target is "reasonably believed to be outside of the United States." Instead, the attorney general would have power to authorize NSA surveillance of foreign targets and to compel Internet and telecommunications companies in the United States to comply with requests for data or access to the communications flowing through their networks.
That provision has prompted significant resistance from Democrats, many of whom have been calling for the resignation of Atty. Gen. Alberto R. Gonzales over allegations that he misled Congress or lied to lawmakers in testimony about NSA surveillance activities.
In an apparent concession to those concerns, the White House modified its proposal late Wednesday to include the national intelligence director in the approval process and to allow the FISA court to review certain activities.
Gonzales made a new attempt Wednesday to tamp down the controversy over his testimony, in which he has repeatedly said there were never serious disagreements within the Bush administration or the Justice Department over the warrantless surveillance program. His statements appear to be contradicted by testimony from other officials, including FBI Director Robert S. Mueller III, who said the program had raised serious concerns.
Gonzales acknowledged Wednesday that some NSA activities authorized by Bush after Sept. 11, 2001, "did precipitate very serious disagreement" within the administration. But he stood by remarks that the agency's warrantless tracking of international phone calls and e-mail of terrorism suspects — the activity the administration has described as the Terrorist Surveillance Program — did not generate serious dissent.
"This is not to say that the legal issues raised by the Terrorist Surveillance Program were insubstantial," Gonzales wrote in a letter to Sen. Patrick J. Leahy (D-Vt.), chairman of the Senate Judiciary Committee. "It was an extraordinary activity that presented novel and difficult issues and was, as I understand, the subject of intense deliberations within the Department."