The agency that owns the site of the World Trade Center in New York says it is suing Saudi Arabia for damages suffered on 11 September, 2001.
Commemorations begin three years to the minute after the attacks
The Port Authority of New York and New Jersey lost 84 employees in the attacks against the Twin Towers.
The agency did not explain why it held the Saudis responsible, but said it wanted to "preserve its legal options".
A statute of limitation on 9/11 lawsuits expires on Saturday - the third anniversary of the attacks.
The Port Authority said it had "a responsibility to the millions of people who live and work in the region as well as to our bondholders to pursue every legal avenue to recover the losses we sustained".
The bi-state agency leased the World Trade Center to developer Larry Silverstein.
The Port Authority said its action was in line "with similar suits already filed by other injured parties".
Last week the bond brokerage firm Cantor Fitzgerald - which lost more than 650 employees in the attacks, more than any other firm - sued Saudi Arabia, saying it had aided the attackers.
Fifteen of the 19 hijackers were Saudi nationals, and the Riyadh government has been accused of helping al-Qaeda through charities.
In recent years the Saudis have co-operated with the US war on terror.
Saturday's commemorations of the attacks will begin at 0846 (1246 GMT) - the exact time the first plane struck.
In what has become an anniversary tradition, the names of the 2,749 victims will be read.
There will be three more minutes of silence at 0903, 0959 and 1029 (1303, 1359 and 1429 GMT) to mark the times when the second plane struck and each tower collapsed.
Last year children recited the names of the victims. This year bereaved parents and grandparents will do so.