Not long after the second tower was hit on September 11th, an NYPD helicopter flew over both towers and got footage of the smoke pouring out. Then after the first tower fell, they got footage from above of the toxic dust filling the streets of Lower Manhattan.
--The 17-minute video was obtained by the National Institute of Standards and Technology, to help with their ongoing investigation.
They start to fly over the towers at :30, you can see crowds of people walking around on the ground at 2:18, and they zoom in on the side of the south tower at 3:50.)
(--At 4:04 they show footage of the dust from the first tower. And at 12:29, they show footage of the second tower falling, but it's from a distance.