Final beam placed on World Trade Center building

World Trade Center Tower 4 has become the first rebuilt structure at the site of the September 11 attacks, after a final steel beam was placed ahead of its inauguration next year.

Developer Larry Silverstein (yellow tie) and Assembly Speaker Sheldon Silver (C) watch as the last steel beam, signed by members of the crews that helped build the tower, is hoisted 977 feet to the top of Four World Trade Center

Following a ceremony, the last steel beam, signed by members of the crews that helped build the tower, is hoisted 977 feet to the top of Four World Trade Center

Construction workers sign the last steel beam 

Clouds are reflected in the glass of Four World Trade Center 

A US flag fluttered from the beam as construction workers hoisted it to the top of the 72-storey building in an emotional ceremony on Monday.

The construction of the 977-foot tower, designed by Japanese architect Fumihiko Maki, began in 2008.

"The topping out of 4 World Trade Center represents another milestone in the effort to create a new, dynamic World Trade Center at the heart of a resurgent Downtown," project developer Larry Silverstein said at the ceremony.

The unfinished skeleton of the nearby Freedom Tower is already the tallest building in New York City and is set to reach a symbolic height of 1,776 feet, the year of US independence.

Some 3,000 people were killed in the attacks on September 11, 2001 when al-Qaeda hijackers flew two planes into the World Trade Center and a third into the Pentagon, while a fourth plane was brought down in a Pennsylvania field after passengers overpowered their attackers.

Today the New York site includes two fountains sunk into the footprints of the former Twin Towers, with the names of the dead inscribed around the edges.