One World Trade Center rises above the lower Manhattan skyline, April 13, 2013 in New York. It is already New York’s tallest building. But when the last pieces of its spire go up to the roof Monday, April 28, the 104-floor skyscraper will be one step away from becoming the highest in the Western Hemisphere. (AP Photo/Mark Lennihan)

NEW YORK — One World Trade Center already is New York’s tallest building.

And when the last pieces of its spire rise to the roof — weather permitting — the 104-floor skyscraper that replaces the fallen twin towers will be just feet from becoming the highest in the Western Hemisphere.

The Port Authority of New York and New Jersey says the spire pieces plus a steel beacon will then be lifted at a later date from the rooftop to cap the building at 1,776 feet.

Installation of the 800-ton, 408-foot spire began in December, after 18 pieces were shipped from Canada and New Jersey.

The spire will serve as a world-class broadcast antenna.

With the beacon at its peak to ward off aircraft, the spire will provide public transmission services for television and radio broadcast channels that were destroyed on Sept. 11, 2001, along with the trade center towers.

Overlooking the Statue of Liberty and Ellis Island, the high-rise is scheduled to open for business in 2014.

The tower is at the northwest corner of the site, which is well on its way to reconstruction with the 72-story 4 World Trade Center and other buildings.

Monday’s 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 ravaged lower Manhattan.

As officials prepared to erect the spire, the office of the city’s chief medical examiner was working in the hidden alley where debris may still contain human remains.

The new tower’s crowning spire is a joint venture between the ADF Group Inc. engineering firm in Terrebonne, Quebec, and New York-based DCM Erectors Inc., a steel contractor.

The world’s tallest building, topping 2,700 feet, is in Dubai.