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New York, Nov 4 (AP) : The silvery skyscraper that rose from the ashes of the September 11, 2001 terror attacks to become a symbol of American resilience has opened for business, as 175 employees of the magazine publishing giant Conde Nast settled into their first day of work there.

The opening of the country's tallest building yesterday, One World Trade Center, marked a symbolic return to a sense of normalcy for the site where the twin towers fell more than 13 years ago.

"The New York City skyline is whole again," said Patrick Foye, executive director of the Port Authority of New York and New Jersey, which owns both the building and the World Trade Center site.

Steps away from the new 1,776-foot (541-metre) tower are two memorial fountains built on the footprints of the decimated towers, a reminder of the more than 2,700 people who died.

Conde Nast, publisher of Vogue, The New Yorker and Vanity Fair, is expected to move in about 3,000 more employees by early next year, eventually occupying 25 floors of the USD 3.9 billion, 104-story tower.

Privately, some Conde Nast employees acknowledged that they were nervous about working in a skyscraper that could again be a terrorist target.

Foye countered that it's "the most secure office building in America." And its chief architect, TJ Gottesdiener, said the high-rise was built with steel-reinforced concrete that makes it as terror attack-proof as possible — much stronger than the original towers that collapsed when the hijacked planes hit.

The stairwells are built with a hardened concrete core, and wider to allow firefighters to move while people exit. The building's mechanical systems are also encased in hardened concrete.