Jefferson argued that the first-of-its-kind raid trampled congressional independence. The Constitution prohibits the executive branch from using its law enforcement powers to interfere with the lawmaking process. The Justice Department said that declaring the search unconstitutional would essentially prohibit the FBI from ever looking at a lawmaker's documents.

The U.S. Court of Appeals for the District of Columbia Circuit rejected that claim. The court held that, while the search itself was constitutional, FBI agents crossed the line when they viewed every record in the office without giving Jefferson the chance to argue that some documents involved legislative business.

"The review of the Congressman's paper files when the search was executed exposed legislative material to the Executive" and violated the Constitution, the court wrote. "The Congressman is entitled to the return of documents that the court determines to be privileged."

The raid was part of a 16-month international bribery investigation of Jefferson, who allegedly accepted $100,000 from a telecommunications businessman, $90,000 of which was later recovered in a freezer in the congressman's Washington home.

Jefferson pleaded not guilty in June to charges of soliciting more than $500,000 in bribes while using his office to broker business deals in Africa. The Justice Department said it built that case without using the disputed documents from the raid.

The court did not rule whether, because portions of the search were illegal, prosecutors should be barred from using any of the records in their case against Jefferson. That will be decided by the federal judge in Virginia who is presiding over the criminal case.

"Today's opinion underscores the fact that the Department of Justice is required to follow the law, and that it is bound to abide by the Constitution," defense attorney Robert Trout, said, promising more legal challenges to "overreaching by the government in this case."

The Justice Department did not immediately return messages seeking comment on the decision. Officials have said they took extraordinary steps, including using an FBI "filter team" not involved in the case to review the congressional documents. Government attorneys said the Constitution was not intended to shield lawmakers from prosecution for political corruption.

The court was not convinced. It said the Constitution insists that lawmakers must be free from any intrusion into their congressional duties. Such intrusion, even by a filter team, "may therefore chill the exchange of views with respect to legislative activity," the court held.

The case has cut across political party lines. Former House Speakers Newt Gingrich, a Republican, and Thomas Foley, a Democrat, filed legal documents opposing the raid, along with former House Minority Leader Bob Michel, a Republican.

Conservative groups Judicial Watch and the Washington Legal Foundation were joined by the liberal Citizens for Responsibility and Ethics in Washington in supporting the legality of the raid.

Following his indictment, Jefferson's supporters accused the Bush administration of targeting black Democrats to shift attention from the legal troubles of Republican congressmen.

"We are confident that as this case moves forward, and when all of the facts are known, we will prevail again and clear Congressman Jefferson's name," Trout said Friday.

Despite the looming investigation, Jefferson was re-elected to a ninth term in 2006. His win complicated things for Democratic leaders who promised to run the most ethical Congress in history.

House Speaker Nancy Pelosi, D-Calif., stripped Jefferson of his seat on the powerful Ways and Means Committee and placed him instead on the Small Business Committee. He resigned that committee assignment after being indicted.

The case was considered by Chief Judge Douglas H. Ginsburg, Judge Karen Lecraft Henderson and Judge Judith W. Rogers.