The National Park Service keeps the register, which now lists nearly 82,000 places. It does not ordinarily consider a property whose significance goes back less than a half-century, unless what happened there was of exceptional importance. (An example in Manhattan is the Stonewall Inn on Christopher Street, often called the birthplace of the gay civil rights movement because of an uprising by bar patrons during a police raid in 1969. It was added to the register 30 years later.)

In the case of the trade center, the agencies said there was already more than enough evidence of the exceptional importance of the terrorist attacks in United States history, including the "deaths of an unprecedented number of individuals in a single location resulting from foreign attacks on American soil." Kevin M. Rampe, the president of the Lower Manhattan Development Corporation, said the determination reflected his agency's overall approach to the site. "From the beginning, we've been committed to ensuring that the historic nature of the site is not only recognized, but preserved," Mr. Rampe said, "while at the same time restoring the site to its historic role as a center of commerce, as it was on Sept. 11."

While almost no one would dispute the historic nature of the attack, there has been considerable uncertainty and debate over the significance of the physical remains. Three months ago, in the face of a growing preservation campaign, the corporation required the finalists in the memorial competition to ensure that their designs would provide future visitors with access to the lowermost foundations of the twin towers. Earlier rules said only that the tower outlines, or footprints, should be made visible in the memorial design.

Remnants of the enormous steel columns that define the footprints were among the existing features highlighted in the new document. It also noted the presence of:

¶The towers' interior columns and elevator pits, pumps and drainage lines.

¶A heavily damaged stairway and escalator structure that once led up from Vesey Street to the trade center plaza and a pedestrian bridge leading to 7 World Trade Center.

¶Vestiges of the 95-year-old terminal of the Hudson & Manhattan Railroad, predecessor of the PATH system, including two cast-iron tubes through which trains ran.

¶Parts of six floors of the underground parking garage, where smoke scars from the attack are still visible, and openings in the foundation wall that once led to the garage from ramps in the middle of West Street.

¶Holes for two pairs of pipes, with diameters of 60 inches and 66 inches, that were used to pump water between the Hudson River and the trade center's cooling system.

"Physical features surviving on the site that have structural, functional and material integrity retain their quality of association with the profound events of that day, as well as the post-Sept. 11 recovery effort," the document said. "Although buildings and infrastructure within the W.T.C. were destroyed and their ruins removed, the physical environment surrounding the W.T.C. site remains essentially as it was on Sept. 11."

The review is required by the National Historic Preservation Act for projects that receive federal financing. These include the permanent PATH terminal, the rebuilding of West Street-Route 9A and the overall planning by the development corporation, which is supported by a grant from the Department of Housing and Urban Development. More than 60 "consulting parties" - including relatives of those who died in the attack, city and state officials, three United States representatives, planners and architects, property owners and tribal representatives - are also participating.