The destruction of the World Trade Center by two hijacked planes on 11 September 2001 counts as a single attack, a jury has ruled.
The court case could decide funding for the Freedom Tower
The attacks killed more than 3,000 people, and destroyed the Center's iconic twin towers.
Developer Larry Silverstein had argued that two events had occurred, each attracting $3.5bn worth of insurance.
But the jury said otherwise, in a move which could reduce the money available for rebuilding by at least $1bn.
The jury's decision is only the first of many covering the question of which insurers owe how much to Mr Silverstein.
While a group of Lloyd's of London insurers are the beneficiaries of Thursday's ruling, they account for only $1bn of the $3.5bn for which the towers were insured.
The eight members of the Lloyd's consortium had all signed a broker's form which defined both towers' destruction as a single event.
But the remaining trials could still see Mr Silverstein - whose lease was only six weeks old when the towers fell - collect twice from the remaining insurers.
The case could decide how much money is available for the ambitious rebuilding plan, designed by Daniel Libeskind and David Childs.
The new plan is centred on a skyscraper to be called the Freedom Tower.