evidence: Thousands of people headed for the World Trade Center, a complex of seven buildings that included a pair of skyscrapers known as the twin towers. Each tower had 110 stories and stood about 1,360 feet high. The tallest buildings in New York City at the time, the twin towers rose above the city’s downtown skyline. Nobody there knew that in just a few hours, both buildings would fall. People who live in New York are used to seeing and hearing airplanes flying overhead. But on the morning of September 11, people stopped on the streets and looked up. The sound of an approaching airplane was too loud, and the plane seemed to be flying too low. To the horror of people watching below, the airplane flew straight into one of the twin towers of the World Trade Center. [...] Back in New York City, dark smoke poured from the twin towers. People rushed to escape the area, which later became known as ground zero. First responders—including police officers, firefighters, and paramedics—arrived within minutes of the first attack on the World Trade Center. They rushed into both towers to help people trapped inside, even though it would be an extremely difficult rescue operation. Almost all the elevators in the twin towers had stopped working. So rescuers started climbing up the stairs, but many were blocked by rubble or fire. Still, firefighters forged ahead, ignoring the danger. (Read more about the heroes of 9/11.)