Construction on the Freedom Tower could come to a grinding halt less than a month after it started if insurance companies decide to stop payments in light of the most recent deal struck between Larry Silverstein and the Port Authority.

“The importance of the insurance companies ensuring their obligation is total,” Mr. Silverstein said yesterday. “Without the proceeds, without Liberty Bond financing, there is no way you can rebuild the trade center.”

A delay in settling the question of insurance payouts would threaten the agreement reached on April 26 – after months of bickering and failed negotiations – between Mr. Silverstein, who holds a 99-year lease on the former World Trade Center site, and the Port Authority of New York and New Jersey, which owns the 16-acre site.

Under the agreement, Mr. Silverstein forfeited his rights to lease out the Freedom Tower and another tower south of ground zero. As part of that exchange, the Port Authority would receive about 35% of future insurance payments – worth hundreds of millions of dollars – related to the terror attacks of September 11, 2001.

Mr. Silverstein would retain the right to construct three commercial towers at ground zero and retain about 57% of the insurance proceeds. The remainder of the proceeds would be dedicated to building retail at the site. Silverstein Properties was expected to receive an estimated $3.2 billion more in insurance money from the total award of $4.6 billion.

The agreement requires that a clarification of the insurance claims be settled by September for the deal to move forward. A lengthy court battle could severely delay the delicate timeline set out in the deal.

Silverstein Properties and the Port Authority sent a joint letter on May 15 to the heads of 16 insurance companies, asking for assurance that they would pay out the claims under the new agreement. The letter gave the insurers until Monday to respond.

The transfer of hundreds of millions of dollars in insurance money is presumably going to be scrutinized by the insurance companies who are still battling Mr. Silverstein in court over the amount of the final settlement.

The chairman of the insurance company Swiss Re America, Jacques Dubois, told The New York Sun that the new agreement would require a reevaluation of the terms of coverage.

“There was a conceptual framework that has been agreed to and we need to see details of the final agreement to determine if it affects our coverage obligations,” he said.

Other insurers, including AIG and Chubb Corporation, said their obligations to the World Trade Center site are complete. Representatives of Allianz Insurance Company refused to comment.

An executive of Silverstein Properties, John Lieber, said the terms of the insurance policy named the Port Authority and Silverstein Properties as co-insurers, like “husband and wife,” and that the insurance payouts should be transferable from one entity to the other.

If the companies do refuse payment, the executive director of the Port Authority, Kenneth Ringler, said yesterday that the Port Authority and Silverstein Properties would file a lawsuit to obtain the money. But Mr. Ringler expressed optimism that the insurance companies would go along with the new arrangement.

“I don’t expect they will say no,” he said.

A spokeswoman for Governor Pataki yesterday said the governor urged the insurance company to continue making their payments.

The announcement about the insurance question came yesterday during Mr. Silverstein’s testimony at a hearing in front of a state Assembly committee, and state speaker, Sheldon Silver.

Leaders from the city, the state, and the Port Authority also testified during the day-long event. Most officials read from lengthy scripted reports updating the progress of rebuilding Lower Manhattan. The panelists submitted to questioning by Mr. Silver and Assemblyman Richard Brodsky, a Democrat of Westchester, over the slow pace of progress and details of the recent agreement.

Despite an outpouring of confidence and optimism by those that testified,the questioning exposed cracks and raised questions on the potential to reach the ambitious timeline under which building would be completed by 2012.

“My analysis of it is an agreement to agree, basically, with a lot of questions,” Mr. Silver said.

“I wouldn’t have another groundbreaking,” Mr. Brodsky said.

Deputy Mayor Daniel Doctoroff, the city’s leading development figure, said he was “90% plus” confident that the buildings would be built on schedule. He called the deal, “unusual in virtually every single way. We have broken barriers in many cases.”

Messrs. Brodsky and Silver questioned officials about specific provisions that could trip up the April deal, which must be finalized by September. Outstanding points include the transfer of the retail development rights to Silverstein Properties from developer Westfield America, and where the state will find government tenants for 1 million square feet of office space. Mr. Brodsky wondered aloud whether a large proportion of government tenants would affect development and the return on investment.

“It sounds like a lot of the revenue stream here is taxpayer revenue for government office space,” Mr. Brodsky said.

Messrs. Brodsky and Silver asked for more detailed submissions from the state, the city, and the Port Authority about the use of more than $20 billion in federal funds given to New York City following the terrorist attacks of September 11, 2001.