The organization then leapfrogged to attack Western targets, including two U.S. embassies in East Africa, the U.S. destroyer Cole, and finally the World Trade Center and the Pentagon.

"In a lot of ways this case is very unique, and in a lot of ways it is very mundane," Cozen said in an interview. "It is unique in that it is grounded in one of the worst events in U.S. history. It is mundane in that we are not breaking new ground in tort [liability] law. The law has always recognized the liability of those who participate in a conspiracy and those who aid and abet."

Cozen is suing under the 1976 Foreign Sovereign Immunities Act, which protects foreign governments from being sued by U.S. citizens except in rare circumstances. While the standard is extremely high, federal courts have permitted lawsuits in cases where foreign countries engaged in criminal conduct, such as murder.

Even if Cozen loses the appeal and the Saudis retain immunity, U.S. District Judge Richard Conway Casey ruled that there is enough evidence to proceed against several Islamist charities, banks, and alleged terrorism financiers named in the lawsuit.

While Cozen was the first, a half-dozen other groups have sued the Saudis to hold them liable for supporting Islamist charities allegedly tied to al-Qaeda.

Among the other plaintiffs: the estate of an FBI agent killed in the 9/11 attacks on the World Trade Center, and the investment firm Cantor Fitzgerald, which lost 657 employees when American Airlines Flight 11 slammed into the north tower.

Outside the courtroom, the Cozen lawsuit has caused friction between the United States and Saudi Arabia, which, after Israel, is this nation's most important Mideast ally.

In a filing with the Second Circuit, Saudi Arabia said the lawsuit had undermined the nations' ability to work together to fight terrorism.

"This concern is felt in all circles of the Saudi government," said Nizar bin Obaid Madani, Saudi minister of foreign affairs.

The litigation, Madani said, "sends a confusing and mixed message about the relationship between the governments of the United States and Saudi Arabia."

The question of whether elements of the Saudi government offered support to jihadists intent on attacking the West has fueled intense and even acrimonious debate at the highest levels of the U.S. government.

Former New Jersey Gov. Tom Kean, who chaired the 9/11 Commission, said he was uncertain. The royal family is so huge - there are thousands of members - that Kean said it was difficult to know how power was distributed and who had authority.

Former Republican Sen. Slade Gorton of Washington, a 9/11 Commission member, is less cautious.

"Clearly, the central moving figures in the 9/11 scandal were Saudi, and clearly that wasn't a coincidence," he said. "The fact that there is a particularly militant and extremely conservative form of Islam that is, in effect, the state religion of Saudi Arabia - well, there has always been tension between the United States and Saudi Arabia over that.

"Do we pull punches with the Saudis on the charities and other matters because they can help us counterbalance Iran, they can help us bring the Palestinians and the Israelis to the table, they can serve as a forward staging area for our military in the Middle East?" Gorton asked. "I don't think there is any question but that is the case."

David E. Long, former deputy director of the State Department's Office on Counterterrorism, who is fluent in Arabic and travels regularly to the Mideast, said he believed it was unreasonable of Americans to expect that before 9/11 the Saudis would have cracked down on terrorism financiers.

They didn't have the forensic skills or financial monitoring tools, said Long, a former lecturer on the Middle East at the University of Pennsylvania. Moreover, until then it was unthinkable to many Saudis that charitable organizations might promote violent international jihad.

One of the central principles of Islam, he said, is Zakat, the giving of alms to the poor, and once the money went out the door, Saudi donors assumed it would be used for charitable purposes.

"Traditionally, there was little or no oversight throughout the economy, trust was highly personalized, and caveat emptor was the order of the day," Long said. "It is . . . my view that there is no basis for the claim that the Saudi government wittingly supports or has continued to turn a blind eye on foundations supporting terrorist operations."

Cozen's case is built on thousands of Treasury Department and law enforcement findings, declassified diplomatic cables, and military and intelligence reports.