The Twin Towers.
When people think of 9/11, they often think of the Twin Towers. The first plane hit the first tower at 8:46am. People across America we're shocked, but nothing was thought of it, except that it could be an accident. Shortly after the first plane hit, the second plane hit at exactly 9:03am. This is what shocked everyone, and we knew this was not an accident. After the second plane hit, the first tower fell. After a while of burning the second tower finally collapsed on itself, and thousands of victims were dead. The police and fire department were able to save many people, but some were too high up to be saved.
Below is news footage of Flight 175 crashing into the second tower.
The collapse of the Twin Towers was a dramatic event for everyone in, and out of New York. Even though they clearly fell because of the crash, many more factors contributed to the fall of the twin towers, such as engineering of the Towers. The airplane impact damaged the columns in the two buildings. The loss of steel from the impact of the plane caused weakening and distortion. Some people think the WTC was defectively designed in its evacuation methods, but it was not, it was just not assumed that it would ever have to be evacuated in that time frame.