Members of the media look north at the Manhattan skyline from the 64th floor of the One World Trade Center tower in New York, November 3, 2014. [Photo/Agencies]

NEW YORK - The silvery skyscraper that rose from the ashes of the Sept 11, 2001 terror attacks to become a symbol of American resilience opened for business Monday, 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, 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-meter) 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 $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, T.J. 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.

"If my son told me he had a job in the trade center Tower 1, I would have no qualms about him being there," Gottesdiener said.

One World Trade Center is 60 percent leased. Its eight-year construction came after years of political, financial and legal infighting that threatened to derail the project.

The area has prospered in recent years. About 60,000 more residents now live in the area - three times more than before 9/11 - keeping streets, restaurants and shops alive even after Wall Street and other offices close for the day.

Still, it's a bittersweet victory.

"The city and the world were watching us, and we had to do it right, to do it better than before," Gottesdiener said. "And we did it, we finally did it."