Letters written by Helen Keller. Forty-thousand photographic negatives of John F. Kennedy taken by the president's personal cameraman. Sculptures by Alexander Calder and Auguste Rodin. The 1921 agreement that created the agency that built the World Trade Center.

Besides ending nearly 3,000 lives, destroying planes and reducing buildings to tons of rubble and ash, the Sept. 11, 2001, attacks destroyed tens of thousands of records, irreplaceable historical documents and art.

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