NEW YORK - The rebuilding of New York's World Trade Center site may take up to three decades to complete because the demand for office space has shrunk with Wall Street's falling fortunes.

The Port Authority of New York and New Jersey, owner of the 16-acre site in Lower Manhattan, on Thursday said it was taking a cautious approach after a study showed that it could take until 2037 to lease all the planned office towers.

The Port Authority wants to avoid building speculative office towers before the market revives, its executive director, Christopher Ward, said.

It would be in a way foolish for us to extend too far into that market, Ward said, after a meeting of the Port Authority's board.

He pledged, however, that the authority's own projects -- the memorial to the nearly 3,000 people who died in the September 11, 2001, attacks, a transit hub, and the One World Trade Center skyscraper originally known as the Freedom Tower -- will be completed on time.

The Port Authority commissioned a study of the site's prospects after private developer Larry Silverstein, who is building three office towers there, asked the agency to guarantee some of his bank loans; the Port Authority would own the buildings if Silverstein could not repay the loans

Mr. Silverstein is asking the public sector to essentially finance his buildings, Ward said.

Janno Lieber, who leads Silverstein's World Trade Center team, disagreed, saying the Port Authority was only being asked to backstop the developer's bank loans.

This makes good business sense for the Port Authority, since the towers will pay ground rent to the Port Authority for another 90 years, Lieber said.

The rebuilding has already been delayed by several years due to court fights over the insurance money owed Silverstein and clashes over designs and security.

The Port Authority, struggling with a complex construction site, has also missed deadlines for finishing the preparatory work so that Silverstein can build his three towers.

The memorial will only be partly finished in time for the 10th anniversary. The transit hub could miss its 2013 deadline unless two of Silverstein's towers that bracket it are at least partly erected, due to the site's complicated underpinnings.

The study, carried out for the Port Authority by real estate services firm Cushman & Wakefield, found that the last of Silverstein's towers would not be completely leased until 2037. Having empty office towers looming over the region would further destabilize the market, Ward said.

The study found that Silverstein's Tower 4 would be the first to be fully leased, though not until 2014.

One World Trade Center, which the Port Authority is building, would not be fully occupied until 2018, while Silverstein's Tower 2 would not be completely rented until 2025. The developer's Tower 3 would not meet be fully leased until 2037, the study found.

Lieber said the agency's outlook was too gloomy, though the credit crunch is freezing real estate projects around the globe.

Our view -- and that of city leaders and many other experts -- is that New York will bounce back strongly over the next five years, while we are building these buildings, he said.

Silverstein wants to replace all of the hundreds of thousands of square feet of offices that were destroyed just two months after he leased the World Trade Center. But Ward noted that critics of the Twin Towers say its enormous size was a major drag on the office real estate market for decades.

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