See video from Chopper 4 as the spire is raised to the roof of One World Trade.
The final sections of the World Trade Center spire, adorned with an American flag, were hoisted to the roof of the skyscraper on Thursday, where they will eventually be raised to make the building soar to a symbolic 1,776 feet.
WATCH: Live World Trade Center Cam
As construction workers and onlookers applauded, the last two pieces were lifted by crane to a temporary platform Thursday and will be installed at a later date.
The spire, which altogether extends 408 feet and weighs 758 tons, will serve as a broadcast antenna, providing public transmission services for television and radio broadcast channels that were destroyed in the 9/11 attack.
An LED beacon at the top of the spire will be visible up to 50 miles away on a clear day, according to the Port Authority.
The building, which stands 104 stories and will feature 3 million square feet of office space, is scheduled to open for business in 2014. An observation deck is planned for floors 100 to 102.
The celebration of the reconstructed trade center comes days after a grisly reminder of the terror attack that took nearly 3,000 lives: the discovery of a rusted piece of airplane landing gear wedged between a nearby mosque and an apartment building — believed to be from one of the hijacked planes that hit the towers.
No human remains were found in the alley. The airplane part was removed Wednesday and taken to Brooklyn.
The world's tallest building, topping 2,700 feet, is in Dubai.