World Trade Center

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A New York City firefighter looks at the last column recovered at the World Trade Center site at the dedication ceremony for the National 9/11 Memorial Museum on Thursday, May 15, 2014 in New York. The museum memorializes the September 11 attacks on the World Trade Center in New York. (AP Photo/John Angelillo, Pool)

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A New York City firefighter looks at the last column recovered at the World Trade Center site at the dedication ceremony for the National 9/11 Memorial Museum on Thursday, May 15, 2014 in New York. The museum memorializes the September 11 attacks on the World Trade Center in New York. (AP Photo/John Angelillo, Pool)

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The twisted remains of a portion of the television transmission tower from the World Trade Center is displayed at the National Sept. 11 Memorial Museum, Wednesday, May 14, 2014, in New York. The museum is a monument to how the Sept. 11 terror attacks shaped history, from its heart-wrenching artifacts to the underground space that houses them amid the remnants of the fallen twin towers' foundations. It also reflects the complexity of crafting a public understanding of the terrorist attacks and reconceiving ground zero. (AP Photo)

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The twisted remains of a portion of the television transmission tower from the World Trade Center is displayed at the National Sept. 11 Memorial Museum, Wednesday, May 14, 2014, in New York. The museum is a monument to how the Sept. 11 terror attacks shaped history, from its heart-wrenching artifacts to the underground space that houses them amid the remnants of the fallen twin towers' foundations. It also reflects the complexity of crafting a public understanding of the terrorist attacks and reconceiving ground zero. (AP Photo)

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A painter gives a touch-up to a wall inside the Sept. 11 Museum, adjacent to a pair of World Trade Center tridents, that once formed part of the exterior structural support of the east facade of the building, Wednesday, May 14, 2014, in New York. Leaders of the soon-to-open museum are portraying it as a monument to unity and resilience. (AP Photo)

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A flag-draped casket can seen atop a fire truck as the motorcade arrives for the ceremonial transfer of unidentified remains of those killed at the World Trade Center from the Office of the Chief Medical Examiner to the World Trade Center site, Saturday, May 10, 2014, in New York. The remains will be transferred to an underground repository in the same building as the National September 11 Memorial Museum. (AP Photo/Jason DeCrow)

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Family members of victims of the Sept. 11, 2001 attacks tie black gags over their mouths in protest of the transfer of unidentified remains of those killed at the World Trade Center from the Office of the Chief Medical Examiner to the World Trade Center site, Saturday, May 10, 2014, in New York. The remains will be transferred to an underground repository in the same building as the National September 11 Memorial Museum. (AP Photo/Jason DeCrow)

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A flag-draped casket is lifted out of a police vehicle during the ceremonial transfer of unidentified remains of those killed at the World Trade Center from the Office of the Chief Medical Examiner to the World Trade Center site, Saturday, May 10, 2014, in New York. The remains will be transferred to an underground repository in the same building as the National September 11 Memorial Museum. (AP Photo/Jason DeCrow)