The tower has been redesigned more than once

The Freedom Tower is being built on the site of the Twin Towers destroyed in the 11 September 2001 attacks.

Architects redesigned it after police expressed concern that the 1,776-foot (540-metre) building could be vulnerable to truck bomb attacks.

Under new plans it is further from the road and has a strengthened structure.

The redesign calls for a concrete and steel pedestal, clad in ornamental metalwork, and topped by a tower of glass.

The building is capped with a mast incorporating an antenna, which is meant to evoke the torch of the Statue of Liberty.

"This new design reflects a soaring tribute to freedom and a bedrock commitment to safety and security," New York Governor George Pataki said.

Completion delay

The symbolic height - which in feet refers to the year of the American Declaration of Independence - is one of the few features left from the initial plans.

The building's chief architect said he felt better about the new one than the original.

WORLD TRADE CENTER SITE TIMELINE July 2002: Lower Manhattan Development Corporation (LMDC) releases six initial design concepts and, in following months, launches global search for design and planning professionals Dec 2002: LMDC releases nine initial designs and launches outreach campaign to engage public in planning process Feb 2003: Memory Foundations by Studio Daniel Libeskind selected as plan Sept 2003: Release of refined master site plan July 2004: The cornerstone of the new building is laid

Libeskind: Visionary architect

The Freedom Tower's cornerstone was laid last July, but an assessment by the New York Police Department urged architects to rethink some elements of its structure.

Police feared it would be difficult to protect the building against a possible truck bomb because of the heavy traffic along nearby West Street.

Under the new plans, the distance from West Street has been increased from 25 feet (7.6 metres) to an average of 90 feet (28 metres), AP reports.

Rows and hitches

The project has been mired in controversy.

Initial designs for a replacement for the World Trade Center were greeted with disappointment by the public.

In 2003 the plan submitted by architect Daniel Libeskind was selected, but the boldness of his original proposals was later toned down.

A lower insurance payout on the Twin Towers than had been expected by the leaseholder further complicated the project.

And victims' relatives argued that construction would damage the foundations of the Twin Towers, which they regard a piece of American history.

The building is due to be completed by the end of the decade.

The skyscraper is planned to be among the tallest buildings in the world.