Images from a police helicopter of the 9/11 attack in New York have been released for the first time.
They show smoke and ash billowing over Manhattan as the World Trade Centre collapses.
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The photos, from a dramatic new angle, were obtained by ABC News, which filed a Freedom of Information Act request with the National Institute of Standards and Technology (NIST) last year.
NIST collected the images as part of its investigation into the terrorist attack.
ABC said it was provided with 2,779 pictures on nine CDs. Many of the photos, taken from police helicopters, have never been released before.
One photo shows a close-up of the upper floors of the burning towers. Others show the towers' dramatic collapse in a thick plume of smoke and debris and sweeping views of billowing smoke clouds.
Meanwhile structural steel for the 1,776-foot tower that will be known as 1 World Trade Center has risen 200 feet above street level, a tangible sign of ground zero progress.
"This progress is one more sign that the site is not a pit anymore," said Chris Ward, executive director of the Port Authority of New York and New Jersey, which owns the 16-acre trade center site.
Workers are now installing 16 steel nodes on the 20th-floor level, Ward said. They serve as joints between the steel framing for the building's podium and the steel for the rest of the tower.
The 104-story skyscraper is scheduled to be completed in 2013.
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