Gary Hershorn / Reuters The final section of the spire that will top off One World Trade Center is raised past iron workers to the top of the building in New York, on May 2. The spire will be permanently attached at a later date.

Construction crews on Thursday hoisted the final segment of spire that will top One World Trade Center and complete a piece of the New York City skyline missing since the 9/11 terror attacks.

A crane guided the final piece into a temporary structure that will house the section until final installation by iron workers at a later date.

Once installed, the spire — weighing more than 700 tons — will crown the Freedom Tower at 1,776 feet, making it the tallest building in the Western Hemisphere, according to the Port Authority of New York and New Jersey. The building currently tops out at 1,368 feet.

9/11 Memorial President Joe Daniels watched the spire piece rise Thursday morning from the memorial's office windows overlooking the World Trade Center site.

"It's a big milestone in the history of the rebirth of the site," Daniels told NBC News. "This renewal of spirit, to see spring here and this beautiful weather, the memorial fountains and the flag on the spire piece going up. It was one of those things that you won't forget."

By chance, the hoisting fell exactly two years after Navy SEALS shot and killed Osama Bin Laden, the mastermind behind the terror attacks that demolished the two World Trade Center towers and killed thousands of Americans.

"To have the One World Trade Center spire happen today — it feels poetic, and it feels like poetic justice," Daniels said.

The event came one day after a 250-pound piece of an airplane wing, believed to be part of a 9/11 jetliner, was removed out of an alley near the World Trade Center where it was found last week and taken into police custody.

When the building is completed, and once it is verified by the Council on Tall Buildings and Urban Habitat, One World Trade Center will be the third-highest building in the world, behind Dubai's Burj Khalifa (2,717 feet) and Mecca's Makkah Royal Clock Tower Hotel (1,972 feet).

Willis Tower (formerly called Sears Tower) and the Trump International Hotel & Tower, both in Chicago, are currently the two highest buildings in the U.S.

The spire — complete with galvanized steel broadcast rings — will serve as part of the One WTC's transmission facilities for the region's media outlets. Perched at its tip is the spire's stainless steel beacon.

A large group of spectators gathered to watch the needle's ascent on Thursday.

Tourist Moulen Katherine called the spectacle "impressive" and "emotional."

Rick and Cindy Baldwin of Charlotte, N.C., had just arrived in New York City and called the sight "inspiring" and felt "excited to be part of New York City."

"I'm more interested in watching the people, they are as inspiring as the spire — the camaraderie they feel as they watch it rise... we get to be part of history," Cindy Baldwin said.

Pockets of workers and tourists watched as two building crew members affixed an American flag to the spire.

Just outside the scrum, two building crew members, clad in white hard hats and neon yellow safety vests, called to each other from the sidewalk: "How you doing — all right?"

"Yeah, just trying to get a good picture," said the other as he raised his phone for a photo.

The spire's pieces were scheduled to be raised Monday but were delayed by wind.

Mark Lennihan / AP Workers attach a harness to the final piece of spire before it is hoisted to the roof of One World Trade Center, on May 2.

Justin Lane / EPA People watch as the spire is hoisted to the top of One World Trade Center in New York, on May 2. Once the spire is put in place One World Trade Center will be the tallest building in the western hemisphere.

Lucas Jackson / Reuters An ironworker uses a line to steady the final piece of a spire, affixed with a U.S. flag, before it is lifted to the top of One World Trade Center in New York, on May 2.

Justin Lane / EPA Workers watch as the spire for the top of One World Trade Center is hoisted to the top of the building in New York, on May 2.