The legal wrangle over whether the destruction of the World Trade Centre (WTC) was the result of one event or two heated up this week.
A New York judge ordered lenders who helped finance the WTC to give evidence.
The decision will be seen as a victory for Swiss Re, the lead insurer involved.
Swiss Re contends the tragedy was the result of one co-ordinated attack and therefore one insured event.
Leaseholder Larry Silverstein contends the towers' destruction constitutes two insured events.
If the court agrees, this would increase the claim from $3.55bn (£2.29bn) to $7.1bn (£4.6bn).
The decision follows a similar one last month requiring Willis, the broker that placed cover for Silverstein, to give evidence.
Swiss Re's legal team welcomed the ruling.
Silverstein Properties made no comment on the case.
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