This photo taken Sept. 11, 2001 by the New York City Police Department and obtained by ABC News, which claims to have obtained it under the Freedom of Information Act, shows smoke and ash engulfing the area around the World Trade Center in New York. (AP Photo/NYPD via ABC News, Det. Greg Semendinger)

The photos, taken from a helicopter by New York Police Det. Greg Semendinger, show the twin towers in flames, falling amid huge, billowing clouds of dust, debris and smoke that envelop downtown Manhattan.

While other photographs of the falling towers were taken on Sept. 11, 2001 from satellites, rooftops and other aircraft, these photos are the only ones known to have been taken up close from an aircraft flying overhead in city airspace.

"With these pictures, you move from a specific slice of information to much greater context," said Jan Ramirez, chief curator at the National September 11 Memorial & Museum.