Crane operators hoisted the final pieces of the spire atop One World Trade Center on Thursday, helping to fill the void in the New York City skyline left by the attacks of September 11, 2001.
Once a beacon is installed, the tower will stand 1,776 feet high, making it the tallest building in the Western Hemisphere, though still well short of the world's tallest structure.
Ironworkers will finish installing the spire at a later date.
The tower is one of four skyscrapers designed to rise around the site of the fallen Twin Towers in a partnership between developer Larry Silverstein and the Port Authority of New York and New Jersey, which owns the site.
The footprints of the fallen towers have been turned into a memorial to the nearly 3,000 people killed in the 2001 attacks by al Qaeda hijackers.
One World Trade Center, formerly called the Freedom Tower, will eclipse what has been the tallest building in the Western Hemisphere, Chicago's Willis Tower, at 1,451 feet.
By one measure, however, the Willis Tower still holds title as the tallest building in America. Counting its antenna, the Willis Tower reaches 1,729 feet. One World Trade Center stood at 1,701 feet on Thursday, and the beacon will add another 75 feet, a Port Authority spokesman said.
The Council on Tall Buildings, an arbiter of claims over building heights, has yet to review One World Trade Center, said Daniel Safarik, editor of the group's publications. The council recognizes the lower of the two figures for the Willis Tower, he said.
The United States was home to the tallest building in the world for much of the 20th century, but has been surpassed by the Middle East and Asia.
The tallest building in the world currently is Dubai's Burj Khalifa, completed at 2,717 feet.
One World Trade Center ranks seventh on a list of the world's tallest buildings under construction, according to Emporis, a database for building information.
The tallest would be the Kingdom Tower under construction in Jeddah, Saudi Arabia, at more than 3,280 feet, or nearly twice the height of One World Trade Center.
Four other buildings under construction in China and one in South Korea would be taller than One World Trade Center, according to Emporis.