The ruling allows the Justice Department to get wiretap authority in a secret intelligence court, under more relaxed standards than used in a regular criminal court, even if the investigation is not exclusively concerned with terrorism.
Under old rules, the intelligence court could only grant wiretap warrants in cases where terrorism was the sole focus.
In the 56-page opinion overturning a May decision by the ultra-secret Foreign Intelligence Surveillance Court, the three-judge appeals panel said the expanded wiretap guidelines sought by Attorney General John Ashcroft under the new USA Patriot Act law do not violate the Constitution.
The special panel from the U.S. Circuit Court of Appeals for the District of Columbia ordered the lower court to issue a new ruling giving the government the powers it seeks.
The lower court had ruled in a May 17 decision that the USA Patriot Act did not justify the use of certain investigative techniques.
Congress last year passed and President Bush signed the Patriot Act, which among other things loosened standards for obtaining warrants. Prior to the passage of the new law, government officials had to prove their primary purpose for monitoring was foreign intelligence.
Prosecutors in criminal cases must meet higher legal standards to win approval for searches or wiretaps than in intelligence cases.
In March, Ashcroft, in a memorandum to FBI Director Robert Mueller and senior Justice officials, made it easier for investigators in espionage and terrorism cases to share information from searches or wiretaps with FBI criminal investigators.
But the surveillance court, which considers federal search and wiretap requests in secret under the 1978 Foreign Intelligence Surveillance Act, found that Ashcroft's rules could allow misuse of information in criminal cases.
In the May ruling, the secretive court, which approves spying on terror suspects, said that it had been misled dozens of times by the FBI and Justice Department officials.
Documents released at the time showed that the court, which has not publicly disclosed any of its rulings in nearly two decades, said FBI and Justice officials in pursuit of search warrants or wiretap authorizations to spy on suspected terrorists had supplied erroneous information to the court on 75 occasions.
The court also said that intelligence gathered about terror suspects had been improperly shared with prosecutors and FBI agents handling criminal investigations.
Because of these alleged improprieties, the surveillance court imposed restrictions on the procedures formulated by Ashcroft. The court said the information-sharing proposal was "not reasonably designed" to safeguard the privacy of Americans.
The Justice Department amended the guidelines and won the court's approval. But the Bush administration then appealed the court's restrictions on the sharing of information between terrorism investigators and criminal investigators.
In the appeal, Ashcroft said the lower court failed to acknowledge that the new law, passed in response to the Sept. 11 attacks, altered the standard lawyers must meet when seeking to monitor a person and share information between criminal detectives and terrorism investigators.
Under the act, Ashcroft argued, federal lawyers may share information and monitor people in cases in which law enforcement is the primary interest. Government lawyers need only show there is a significant foreign intelligence purpose related to the activity, Ashcroft said.
The Patriot Act changed the surveillance law to permit its use when collecting information about foreign spies or terrorists is "a significant purpose," rather than "the purpose" of an investigation.
Critics at the time said they feared government might use the change to employ espionage wiretaps in common criminal investigations.
The American Civil Liberties Union and several other groups had argued that Ashcroft's proposed guidelines would unfairly restrict free speech and due process protections by giving the government far greater ability to listen to telephone conversations and read e-mail.
"No one is questioning the government's authority to prosecute spies and terrorists," said Ann Beeson, litigation director of the ACLU's technology and liberty program. "But we do not need to waive the Constitution to do so."
It's unclear whether the ACLU or other groups will appeal the case to the Supreme Court.