The team quickly ruled out going after the airlines, reasoning that they had no role in providing security. Even if they had, it would have been difficult to prove the airlines could have foreseen that their planes would be used as weapons.

Hijacked, yes. But crashed into buildings? Likely not.

What about the security companies that screened the 19 hijackers? Cozen said there seemed to be no evidence that the airport screeners had deviated in any way from government procedures.

Even though the hijackers were armed with box cutters, the screening companies had done nothing wrong by waving them through. Box cutters at the time could be legally carried onto a plane.

Someone wondered whether Afghanistan could be sued, since the Taliban rulers had hosted Osama bin Laden since 1996.

That seemed possible until lawyers came back with a quick answer: Afghanistan's only internationally recognized government was in exile in Rome and thus had no assets.

But a lawsuit against Saudi Arabia? That might work.

Zeroing in on Riyadh

That group included Elliott Feldman, the head of the firm's subrogation practice, and Sean Carter, who directed much of the strategy. Carter, known for his capacity to master prodigious amounts of information, quarterbacks the case for the firm.

Also on the team were Scott Tarbutton, a young associate who handled many research and legal tasks, and Adam Bonin, who is married to author Jennifer Weiner (In Her Shoes) and has an elegant writing style of his own.

For advice on the appeals, the firm turned to Stephen Burbank, a Harvard-trained law professor at Penn and a leading authority on the Foreign Sovereign Immunities Act, which governs circumstances under which U.S. citizens can sue foreign governments.

What clinched the decision was an avalanche of information from the U.S. Treasury Department naming dozens of suspect Islamist charities, banks, and alleged terrorism financiers as al-Qaeda allies.

Many were based in Saudi Arabia or had prominent Saudis in leadership roles.

Cozen lawyers also had to be sure that such a defendant made financial sense, for the firm and its clients. A lawsuit of this magnitude would cost many millions of dollars. Moreover, the law governing such cases was evolving and uncertain.

And because any lawsuit against such an important American ally would inevitably raise national-security issues, the U.S. government might step in and halt legal action.

Cozen lawyers tallied what the investigation and legal work would cost, what the case might bring in, and what its chances were of succeeding.

Such a calculation followed no formula. It was more an art, a way of knowing from years before juries, what might work in a courtroom.

Saudi Arabia was by far the biggest potential target, politically and financially.

There was also economic and legal logic in naming more obscure players, such as Sudan, which hosted bin Laden from 1991 through 1996.