ederal, state and city officials gathered at the rim of ground zero yesterday to celebrate their cooperation in securing $4.55 billion in federal money to overhaul and update the tangle of subway stations, commuter terminals and other transportation outlets in Lower Manhattan.

But the money still falls short of the $7.5 billion that officials overseeing the rebuilding have long said would be needed to complete crucial transportation projects, leaving questions about which projects might be scrapped or scaled back as the rebuilding effort progresses.

The officials vowed that all of the agencies involved in the rebuilding effort would work together to expedite the financing. Michael Brown, the deputy director of the Federal Emergency Management Agency, said the agency was essentially telling local authorities, "You figure out what is best for you and we are going to help fund that for you."

The transportion projects, which are expected to take three to five years to complete, could still be encumbered by the numerous federal agencies and state and local officials who have some say over how the money is used.

The $4.55 billion allocation, part of the previously announced $21 billion in federal aid that President Bush promised New York for its recovery after Sept. 11, includes $2.75 billion set aside for FEMA and earmarked for use in the World Trade Center site cleanup.

That money will be continue to be administered by FEMA, although it can now be used for transportation projects that expand or reconfigure the subway stations and other infrastructure that existed before Sept. 11. Normally, FEMA money can only be used to replace whatever was destroyed in a disaster.

The remaining $1.8 billion discussed yesterday is money whose specific use for transportation projects was approved earlier this month, when President Bush signed a supplemental appropriations bill.

"We will have a multimodal center down here that is worthy of the 21st century," Gov. George E. Pataki said at a news conference on Liberty Street, overlooking the pit that now constitutes the trade center site.

As jackhammers and heavy equipment worked on a temporary PATH station in the background, Mr. Pataki said, "This is, I believe, the first time that FEMA has undertaken to grant a local jurisdiction this magnitude of flexibility, and it will allow us, as you hear behind us, to continue with the process of reconstruction."

While most of the talk was of transportation, Senator Hillary Rodham Clinton made note of plans to erect a memorial to the victims of the Sept. 11 attacks. None of the pending decisions about the downtown infrastructure, she said, "undermines or defers the important decisions about the memorial."

State and local officials involved in the rebuilding effort said the projects most likely to be paid for with the $4.55 billion were a new PATH commuter terminal at the World Trade Center site; a new Fulton Street transit center linking the various city subway lines that converge downtown; and an underground concourse to connect the PATH terminal and the transit center that will stretch from the World Financial Center to the area beneath the intersection of Broadway and Fulton Street.

In addition, planned projects include a bus terminal at or near the trade center site; a reconfigured South Ferry terminal for the 1 and 9 subway lines; a connection between those lines and the N and R lines at Rector Street; improvements and expansion of the ferry terminals at the World Financial Center and Hoboken, N.J.; and a study of ways to connect the Long Island Rail Road and the Metro-North Railroad to Lower Manhattan.

The estimated cost of those improvements is $4.1 billion, according to figures put together by the Lower Manhattan Development Corporation.

But the development corporation, the Port Authority of New York and New Jersey and the Metropolitan Transportation Authority had previously proposed spending $2 billion for a West Street tunnel, to route most traffic away from the surface streets near the trade center site, and $1.3 billion for long-term security enhancements in the subway system and at the tunnels and bridges connecting New York and New Jersey.

Once details of each project are sufficiently clear to begin construction, officials said, the various local and state agencies — the Port Authority, the M.T.A. or the State Department of Transportation — will have to apply to FEMA for specific funds. Those items that are not approved by FEMA could be paid for by separate application to the Federal Transit Administration.

Louis R. Tomson, the president of the Lower Manhattan Development Corporation, said officials involved in the rebuilding might have access to additional money for use on transportation projects, including insurance payments on the destroyed portions of the PATH and subway lines.

The development corporation also has access to money through Community Development Block Grants administered by the United States Department of Housing and Urban Development, which Mr. Tomson said could be used for some transportation-related expenses, or through special federal grants set aside for hazard mitigation or other uses.

Some of the previously outlined projects could also be scaled back, Mr. Tomson said. Some of the initial land-use designs for Lower Manhattan called for only a short stretch of West Street to be put underground, rather than the entire length from Chambers Street to Battery Park. And the estimated costs of security upgrades might be lowered as planning progresses, he said.

"We have not sat down and really refined these ideas yet," Mr. Tomson said. "Clearly we have to maximize the dollars that are available. And we also can begin to look to other funds."