The first foundation columns have been laid for the Freedom Tower that will replace the World Trade Center in New York.
There were cheers as the first 31-foot (9m) beam, painted with the American flag, was lowered into place.
NY Governor George Pataki said: "Today the Freedom Tower rises from the ashes of September 11, and the people of New York and the people of America can be proud."
More columns will be set around the perimeter of the new building over the next fortnight, among them two signed by relatives of those killed in the 9/11 attacks.
The skyscraper is intended to fill the void in Lower Manhattan's skyline left by the collapse of the Twin Towers.
At 1,776ft (541m) the tower will be the tallest building in the US, and one of four skyscrapers intended for the site.
The height in feet is symbolic, referring to the year of the American Declaration of Independence.