A ceremony has been held in New York to mark the start of building on the skyscraper which will replace the destroyed World Trade Center.
The final look of the building is uncertain.
A 20-ton granite block was laid as a cornerstone of Freedom Tower, planned to be one of the tallest buildings in the world at 1,776 feet (541m).
Its height reflects the date of US independence, while the ceremony was held on Independence Day.
But the final look of the building, due for completion by 2009, is uncertain.
New York's mayor said he could not imagine a more appropriate day for the ceremony.
"The cause of liberty can never be defeated," Michael Bloomberg said in remarks at Ground Zero.
New York Governor George Pataki said the tower would replicate that "great lady of liberty" - and stand as a "new symbol of American strength and confidence".
The project, designed by visionary architect Daniel Libeskind, has been mired in controversy.
Initial replacement designs put forward by the body which controls the site were greeted with disappointment by the public.
Anger at the perceived blandness of the proposals provoked a second competition to provide a master plan, which was won last February by Mr Libeskind.
His plan envisaged a soaring spire a third of a mile high (541 metres), with hanging gardens and a shaft of sunlight that would penetrate the building each year on the anniversary at the moments when the hijacked planes struck on 11 September 2001.
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
December 2002: LMDC releases nine initial designs and launches outreach campaign to engage public in planning process
February 2003: Memory Foundations by Studio Daniel Libeskind selected as plan
September 2003: Release of refined master site plan
But the boldness of Mr Libeskind's original proposals have been toned down in what BBC arts correspondent Lawrence Pollard describes as a classic collision of commerce, practicality and vision.
The lead architect hired by the leaseholder and developer Larry Silverstein has made changes to Mr Libeskind's design for the tower.
While the height and spire remain, the latest plans replace Mr Libeskind's "gardens in the sky" with windmills in an open area at the top.
"Happily, I am still intensively and fully engaged as the master planner for ground zero," Mr Libeskind wrote in a letter published on 27 June in The New York Times in reaction to an article titled, "The Incredible Shrinking Daniel Libeskind".
'A priceless piece'
The BBC's Stephen Evans in New York says a change of plan was inevitable.
Our correspondent says Ground Zero is many things - a mass grave, a field of memories, a political symbol - but it is also prime real estate at the heart of the financial capital of the world.
Libeskind's design has been toned down
A recent legal judgement ordering a lower insurance payout on the Twin Towers than had been expected by Mr Silverstein has complicated the project.
And victims' relatives argue that construction will damage the footprint, or foundations, of the twin towers and the steel-enforced concrete "slurry wall", which encased them.
"This is a priceless piece of our American history," said Anthony Gardner, a Coalition of 9/11 Families member whose brother was killed.