The Supreme Court has rejected a challenge to the Bush administration's domestic spying program.
The justices' decision today, which does not include any comment explaining why they were turning down the appeal, is the latest setback in attempts to find out details about the warrantless wiretapping program that began after the September 11 attacks.
The American Civil Liberties Union wanted the court to allow a lawsuit by the group and individuals over the warrantless wiretapping.
An appeals court had earlier dismissed the suit because the ACLU cannot prove whether their communications have been monitored.
The government has refused to turn over any information about the closely guarded program, including who has been under surveillance.
ACLU legal director Steven R Shapiro has said his group faces a no-win situation: the government says the identities of people whose communications have been intercepted is secret, but only people who know they have been wiretapped can sue because their line was tapped, Shapiro.
The 9th US Circuit Court of Appeals ruled last year against an Islamic charity that also challenged the program, concluding that a vital piece of evidence is protected as a state secret.
In that case, the Oregon-based US arm of the Al-Haramain Islamic Foundation alleged the National Security Agency illegally listened to its calls. The charity had wanted to introduce as evidence a top-secret call log it received mistakenly from the Treasury Department.
A separate lawsuit against telecommunications companies that have cooperated with the government is pending in the San Francisco-based appeals court.
A US district court also is examining whether the warrantless surveillance of people in the US violates the law that regulates the wiretapping of suspected terrorists and requires the approval of a secret court.