Greg Semendinger spent 35 years with the NYPD, 14 as a helicopter pilot. On the morning of Sept. 11, 2001, he was stationed at a Brooklyn airfield.
"I'd been interested in photography all my life," Semendinger said. "I carried a camera on every flight. … We got a call that a private plane had hit the World Trade Center, jumped into the helicopter, [and] we were up there in a couple of minutes."
Semendinger's partner, Jim Ciccone, was piloting their aircraft. Early reports were that a tiny Cessna might be to blame.
"It looked like it was hit by a lot bigger aircraft than a private plane," Semendinger said. "At the time, we still thought it was only an accident."
But the second plane crash established that it was no accident.
"We knew then that it was a terrorist attack," Semendinger recalled.
Smoke engulfed the twin towers.
"Our primary job was to search the roof to see if anybody made it to the roof - whether they broke down the door, whether they pried it open, or however it happened. I was just hoping that somebody made it up there," Semendinger said.
In under an hour after it was attacked, the south tower would fall.
"We tried to get the message across that the writing is on the wall -- the north tower is probably going to come down," Semendinger recalled. "Everybody should get out."
Thirty minutes later, it also collapsed.
"I could see the whole building going down into the mushroom cloud that was surrounding it," Semendinger said. "And then I could see the cloud spreading out through the streets in lower Manhattan."
"For the people that were trapped up there - it's really a shame; they had no place to go," he said.
Greg Semendinger retired from the New York City Police Department in 2002. He has 240 digital pictures of 9/11.