$7bn question: was 9/11 one attack or two?

More than two years after terrorists brought down the World Trade Centre, a federal jury will begin deciding a $7bn (£3.8bn) question: was this one attack, or two? If the latter, the leaseholder is entitled to $7bn insurance; if not he must make do with half that.

Opening statements begin tomorrow in a trial pitting the WTC developer, Larry Silverstein, against 13 of his insurers. The outcome will determine how much money Mr Silverstein gets to rebuild at Ground Zero.

Mr Silverstein contends that the destruction of the WTC constituted two attacks, because the twin towers were hit by hijacked airliners a little over 15 minutes apart.

He and New York development officials have been counting on the larger figure to rebuild on the site within the next decade. The smaller amount could mean years of construction delays at the site, according to the agency in charge of redevelopment.

"Anyone who's in New York knows that how much we recover from this lawsuit will have an impact on the rebuilding," said Kevin Rampe, president of Lower Manhattan Development Corp. The rebuilding costs have been estimated at about $7.5bn, he said.

The proceedings could last months. Initial rulings have gone against Mr Silverstein. A federal appeals court ruled in September that contracts for three of the insurers define the destruction of the buildings as one event, and lawyers will be allowed to tell jurors about the decision.

In the first stage of the case, a jury must decide whether the 13 insurance companies issued their coverage using a broker's form that would define 11 September as one event.

Most of the insurers used either a form prepared by Willis Group Holdings, which a judge has ruled describes the WTC's destruction as one event; or another form prepared by Travelers Property Casualty Corp, which Mr Silverstein says defines it as two events.

The timing of the attack further complicates the arguments. Mr Silverstein signed a 99-year lease on the twin towers just months before the attack. He took steps to obtain insurance in July 2001, but the contracts had not been formally written when the towers fell. And if the jury decides that any of the insurers are bound by a form that would allow the destruction to be defined as two events, a second proceeding will determine whether the attack really was two events. A separate jury would hear that second trial.

A third stage would determine how much each insurer should pay.

Insurers have so far paid out more than $25bn for property losses and damage stemming from the attack, and estimate total losses at up to $40bn.