One World Trade Center has become the tallest building in New York after overtaking the Empire State Building as the 100th floor is installed.
A steel column was lifted into place to elevate the height of the skyscraper to 1,271ft, 21ft higher than the Empire State Building.
Building began in April 2006 and it will reach a height of 1,776ft when it is completed in autumn 2013.
The skyscraper, dubbed the Freedom Tower, was built to replace the Twin Towers and commemorates the 2,996 deaths from the September 11 attacks.
Laura Trevelyan, a BBC journalist in New York, said people had mixed emotions about the tower.
She said: "It's bittersweet, one construction worker told me. It's a beautiful thing that we're up there, but it's a horrible reason to have to put it up.
"The rebuilding of Ground Zero has been lengthy, fraught with emotion and politics. The main tower was redesigned to make it more secure, losing both its original sleek design and the loftier title of the Freedom Tower.
"But now, all eyes are drawn upward and 1 WTC has reclaimed the title of tallest building in New York. It's also a reminder of all that happened here on 9/11."
Mayor Michael Bloomberg celebrated the milestone. "The New York City skyline is, once again, stretching to new heights," he said.