World Trade Center's painful transformation to Freedom Tower

A view of the lower Manhattan skyline on Feb. 21, 2001 from Brooklyn.

The twin towers of the World Trade Center rise above the New York skyline in this June 23, 1999 photo.

A picture taken 20 May 1986 in New York shows the Statue of Liberty and the twin towers of the World Trade Center in the background.

2001: Rue Guardino stands at the railing at a waterfront observatory in Bayonne, N.J. with her daughter, Olivia Guardino, 5 and son Stephen Mayer, 11 as they view the skyline of lower Manhattan in the aftermath of the attack of the World Trade Center

2001: The second hijacked plane is seen moments before striking the second tower of the World Trade Center on September 11.

2001: The rubble of the World Trade Center smoulders.

2001: A charred document sits in the soot in the area near the destroyed World Trade Center Sept.12.

2001: The first tower of the World Trade Center collapses after being hit by a hijacked commercial aircraft.

2001: A man stands in the rubble, and calls out asking if anyone needs help, after the collapse of the first World Trade Center on Sept. 11.

2001: An overall of the scene at the World Trade Center as rescuers comb the wreckage.

2001: Smoke lingers in the air as a new day dawns over Manhattan, one week after a terrorist attack destroyed the World Trade Center in New York City.

2002: Ground Zero as seen from the roof of the Stage Door Deli on Vesey Street in lower Manhattan.

2002: The Last Load ceremony marks the official end of the recovery effort. The ambulance carrying the symbolic empty stretcher and the Last Beam proceed up West Street past the site.

2002: An all-agency honor guard, family members and dignitaries look on as the recovery truck carrying the last steel beam moves up the ramp from inside of the World Trade Center site as the recovery officially ends in New York, May 30, 2002.

2002: Six months after a terrorist attack destroyed the World Trade Center twin towers in New York City, a "Tribute in Light" was lit in Manhattan.

2003: Family members gather on the second anniversary of the World Trade Center attacks in New York City.

2003: A view of the part of Ground Zero that is now home to the newly built World Trade Center Path Train Station.

2003: The "Tribute in Lights" soars over the NY skyline from next to ground zero on the second anniversary of 9/11.

2004: Two women embrace during ceremonies marking the third anniversary of the 9/11 terrorist attacks at Ground Zero.

2004: This aerial view shows Ground Zero at the World Trade Center.

2004: Lower Manhattan is viewed from the 13th floor of 7 World Trade Center. 7 World Trade Center, the last tower to topple on Sept. 11, 2001, is the first to be rebuilt.

2005: Two beams of light shine from lower Manhattan in remembrance of the World Trade Center, on the fourth anniversary of the Sept.11, 2001 terrorist attacks in New York City.

2005: The World Trade Center site on the fourth anniversary of the attack that killed 2,749 people.

2006: Families lay flowers into a temporary pool at the footprint of one of the towers at Ground Zero.

2006: The Vesey Street staircase, also known as "survivors stairway" is the last recognizable above ground remnant of the Twin Towers.

2006: Late in the evening, visitors to the World Trade Center site walk around a time line display on the security perimeter five years after the attacks of Sept. 11, 2001. In the background is the newly constructed 7 World Trade Center and to the right is the entrance to the Path Train and city subway lines.

2006: The last remnants of steel support beams that once held up ceilings of the World Trade Center's underground parking garage at 6 World Trade.

2006: A view from the northwestern corner looking at the construction at the World Trade Center site where the concrete footings for the new Freedom Tower are being prepared.

2007: Family members of the victims of the attacks on the World Trade Center place flowers at a temporary reflecting pool at Ground Zero on the sixth anniversary of the attacks.

2006: Families and friends of victims of the 911attacks surround a reflecting pool to mark the sixth anniversary of the Sept. 11 terror attacks.

2007: Construction continues at the World Trade Center site ahead of the sixth anniversary of the terrorist attacks.

FILE - This Sept. 8, 2008 file photo shows the early stages of reconstruction starting at the World Trade Center site in New York. Thirteen years after 9/11 forever changed the New York skyline, officials say developments at the World Trade Center are on track and on budget. (AP Photo/Mark Lennihan) ORG XMIT: NYML301

2008: Cranes at left in position on the construction site for the foundation for One World Trade Center in New York on the seventh anniversary of the Sept. 11 terrorist attacks.

2009: The World Trade Center construction Sept. 11, 2009.

2009: A construction crane works above the foundations of World Trade Center Tower 4.

2010: The south memorial pool under construction, with the Freedom Tower behind, under construction.

2011: A New York City police officer salutes at the North Pool of the 9/11 Memorial during the singing of the National Anthem at the tenth anniversary ceremonies in N.Y.

2011: Families visit the North Memorial Pool, with One World Trade Center under construction during tenth anniversary ceremonies at the site of the World Trade Center Sept. 11, 2011.

2011: 1 World Trade Center, formerly called the "Freedom Tower", and the 9/11 Memorial and Museum.

2011: Families visit the south memorial pool during tenth anniversary ceremonies at the site of the World Trade Center Sept. 11, 2011.

2011: Families visit the north memorial pool during tenth anniversary ceremonies at the site of the World Trade Center.

2011: A view of a rose on the names inscribed around the north pool of the 9/11 Memorial during tenth anniversary ceremonies at the site of the World Trade Center.

2011: A woman cries while looking at the names inscribed at the north pool of the 9/11 Memorial.

2012: Tribute in Light shines as One World Trade Center, right, rises under construction on the eleventh anniversary of the terrorist attacks on lower Manhattan.

2012: Christine Gonda places a picture of firefighter George Kane in the engraving of his name at the south pool during memorial ceremonies for the eleventh anniversary of the terrorist attacks on lower Manhattan.

2012: People stand by a memorial pool during memorial ceremonies for the eleventh anniversary of the terrorist attacks on lower Manhattan.

2013: Showkatara Sharif, of Chantilly, Va., stands next to the engraving of her daughter's name, Shakila Yasmin, at the edge of the North Pool at the 9/11 Memorial during memorial ceremonies for the twelfth anniversary of the terrorist attacks on lower Manhattan.

2013: Victims' family members view names along the south reflecting pool at the 9/11 Memorial during ceremonies for the 12th anniversary of the terrorist attacks on lower Manhattan at the World Trade Center site.

2013: One World Trade Center towers over lower Manhattan in New York City. The building was deemed the tallest building in North America, snatching the title previously held by Willis Tower in Chicago (previously titled Sears Tower).

2014: The south memorial pool opens at the site of the World Trade Center, with the 9/11 Memorial Museum at rear right, and One World Trade Center rising above.

2015: The view from the Staten Island Ferry of southern Manhattan.

2015: One World Trade Center soars above the skyline of southern Manhattan.

2015: The National September 11 Memorial and Museum, center foreground, surrounded by One World Trade Center, left, the white v-shaped transportation hub, center, and 4 World Trade Center, the tall building at right, in New York. The building under construction at center right is 3 World Trade Center. Sept. 11, 2015 will mark the 14th anniversary of the 9/11 terrorist attacks on the United States.