The end came 1 hour, 42 minutes, 5 seconds later when that tower fell in a heap of rubble and cement dust.
In the interim on Sept. 11, 2001, United Airlines flight 175 crashed into the south face of the south tower at 9:02.54, dooming it to collapse 56 minutes, 10 seconds later.
Seven hours after that, at 5:20.33 p.m., the fire-damaged, 48-story building 7 also fell, completing the destruction of the complex.
Although initial reports varied from one reporting agency to another, the precise moments eventually were established by a seismograph at Columbia University's Lamont Doherty Earth Observatory in Palisades, N.Y., 21 miles north of the World Trade Center.
Lamont-Doherty also recorded the impact, according to the Richter Scale that measures earthquakes. Each full digit represents a 10-fold increase in a tremor's magnitude.
Here are the Eastern Daylight times, duration of impact and Richter scale magnitude:
- 8:46.26: American Airlines flight 11 hits north tower, 12 seconds, 0.7.
- 9:02.54: United Airlines flight 175 hits south tower, 6 seconds, 0.9.
- 9:59.04: South tower collapses, 10 seconds, 2.1.
- 10:28.31: North tower collapses, 8 seconds, 2.3.
- 17:20.33: WTC building 7 collapses, 18 seconds, 0.6.
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