These photos were taken by ex-NYPD Aviation Unit Detective Greg Semendinger from a police helicopter searching for survivors on the rooftops.
Semendinger, retired in 2002 after 35 years, said that he has thought about publishing his work from those days. He added "I almost didn't realize what I was seeing that day. Looking at it now it's amazing I took those pictures. The images are stunning."
The pictures below show a very sad and dark New York City engulfed in thick smoke and the towers under fire just before collapsing.