The owners of ground zero, locked in a new round of heated talks with a private developer about how and when to build office towers at the World Trade Center site, have proposed indefinitely putting off two of three planned skyscrapers until the real estate market recovers, officials familiar with the negotiations say.

One analysis prepared for the Port Authority of New York and New Jersey predicts World Trade Center developer Larry Silverstein wouldn't be able to finish building all three towers he plans for decades, with the last tower finished by 2030.

Silverstein and the Port Authority have been talking on and off for months about rewriting a 3-year-old agreement that gives the developer rights to build three out of five towers planned at the Sept. 11, 2001, terror attack site.

In a failing economy where developers have found it impossible to obtain financing for new projects, Silverstein last fall asked the Port to back financing for two of his towers, three officials told The Associated Press, speaking on condition of anonymity because the talks are confidential.

The Port Authority about a week ago agreed to back about $800 million in financing for one tower already under construction, where the Port Authority has agreed to lease space once it's completed.

The other two towers shouldn't be built until there is enough demand for commercial office space downtown, the Port has said.

An analysis of Silverstein's plan by the Cushman & Wakefield real estate brokerage projected that while two of Silverstein's towers could be built by 2013, a third would not be built until 2030 and fully leased until 2036. A second tower that hasn't been built yet wouldn't be fully leased until 2025, the brokerage says.

The agency says it needs clarity on a rebuilding schedule to prevent delays on other interconnecting projects, like the Sept. 11 memorial and a multibillion-dollar transit hub. All the planned projects on the site have been delayed multiple times since they were announced in 2003.

Port Authority Executive Director Chris Ward said Thursday that even in this market, the government cannot subsidize Silverstein.

"Mr. Silverstein is asking the public sector to finance ... his building," Ward said. Silverstein "is seeking the Port Authority's capacity to finance office space downtown."

Janno Lieber, who oversees the trade center site for Silverstein, said the developer wants to build all the towers as quickly as possible, "as we all promised the public so that downtown could realize its potential."

Silverstein has not formally rejected the port's proposal. Both sides need a new deal, in which Sept. 11 insurance money was split to pay for the towers. Silverstein has paid about $800 million in rent since 2001 for the undeveloped space and has collected over $100 million in development fees. The Port Authority is paying Silverstein $300,000-a-day late fees for not excavating all the land Silverstein needs to build on. The lease requires Silverstein to build his three towers by 2013 or forfeit rights to them.

Kathryn Wylde, chief executive of the Partnership for New York business group, said the real estate market would likely drive the decision how fast to build back office space.

"If you don't have commercial tenants demanding the space, I don't see it being developed," she said. But she wondered at projections like the 2030 date, saying demand for new office space in lower Manhattan would happen long before that.

"It may take five years, it may take 10 years," she said, "but it's not going to take 21 years."