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This Aug. 2, 2012 file photo shows President Barack Obama's message and signature on a steel beam at One World Trade Center in New York. The presidentís words will join those of numerous construction workers at the site who left personal messages on the beams, when they will be sealed behind the facade of the buildings as they progress toward their 2014 completion date. (AP Photo/Mark Lennihan, File)

In this Aug. 2, 2012 file photo, a construction worker signs a ceremonial steel beam at One World Trade Center in New York. The beam was signed by President Barack Obama with the notes: "We remember," ''We rebuild" and "We come back stronger!" during a ceremony at the construction site June 14. The beam, having since adorned with the autographs of workers and police officers at the site, will be sealed into the structure of the tower, which is scheduled for completion in 2014. (AP Photo/Mark Lennihan, File)

This Jan. 15, 2013 photo shows graffiti left by visitors to the World Trade Center on a steel column on the 104th floor of One World Trade Center in New York. Construction workers finishing New York's tallest building at the World Trade Center are leaving their personal marks on the concrete and steel in the form of graffiti. (AP Photo/Mark Lennihan)

In this May 30, 2007 file photo, a steel column for the Freedom Tower bears signatures of Sept. 11 victims' family members at the World Trade Center in New York. Their signatures join those left by some who worked on the towers. Construction workers have left personal messages on One World Trade Center in the form of graffiti. (AP Photo/Mark Lennihan, File)

This Jan. 15, 2013 photo shows graffiti left by Michael Chertoff, the former director of Homeland Security, on a steel column on the 104th floor of One World Trade Center in New York. Construction workers finishing New York's tallest building at the World Trade Center are leaving their personal marks on the concrete and steel in the form of graffiti. (AP Photo/Mark Lennihan)

This Jan. 15, 2013 photo shows a tribute in graffiti to Lilian Fredricks that a construction worker left on a steel column on the 104th floor of One World Trade Center in New York. Fredericks was killed in the 2001 terror attacks. Workers finishing New York's tallest building at the World Trade Center are leaving their personal marks on the concrete and steel in the form of graffiti. (AP Photo/Mark Lennihan)

This Jan. 15, 2013 photo shows drawing of a car on the 104th floor of One World Trade Center in New York. Construction workers finishing New York's tallest building at the World Trade Center are leaving their personal marks on the concrete and steel in the form of graffiti. (AP Photo/Mark Lennihan)

This Jan. 15, 2013 photo shows Spanish graffiti left by a worker on a steel column on the 104th floor of One World Trade Center in New York. Construction workers finishing New York's tallest building at the World Trade Center are leaving their personal marks on the concrete and steel in the form of graffiti. (AP Photo/Mark Lennihan)

In this Jan. 15, 2013 photo, ìAntony," left his graffiti on a steel column on the 102nd floor of One World Trade Center in New York. Workers finishing New York's tallest building at the World Trade Center are leaving their personal marks on the concrete and steel in the form of graffiti. (AP Photo/Mark Lennihan)

In this Jan. 15, 2013 photo, autographs cover a wall on a top floor of One World Trade Center in New York. Construction workers finishing New York's tallest building at the World Trade Center are leaving their personal marks on the concrete and steel in the form of graffiti. (AP Photo/Mark Lennihan)

This Jan. 15, 2013 photo shows graffiti left by workers on a steel column on the 104th floor of One World Trade Center in New York. Construction workers finishing New York's tallest building at the World Trade Center are leaving their personal marks on the concrete and steel in the form of graffiti. (AP Photo/Mark Lennihan)