At 9:35 p.m., the level was at 760 microsieverts per hour; two minutes later it was up to 3, 130 microsieverts.

Nachtwey glanced down, and noticed some people standing on an adjacent roof, looking west and pointing toward the sky.

At 10:29 a.m., Nachtwey heard "what sounded like a waterfall in the sky," and looked up to see the north tower coming right down on top of him.