The 9/11 attacks began at about 8:14 a.m., with the hijacking of Flight 11, and ended when the fourth plane, Flight 93, crashed at 10:03 a.m. That morning, about 3,000 people were killed. In the first 24 hours after the attacks, fewer than 20 survivors were rescued. While many were able to exit the Twin Towers before their collapse, thousands of people inside—including first responders—were trapped or harmed when the buildings fell, resulting in a massive toxic debris pile at Ground Zero. Across the attack sites, a total of 246 passengers and crew members on the four planes were killed, as were over 400 first responders. Firefighters, law enforcement, military personnel, and social workers rushed toward the attacks in an effort to save lives and assist with those injured. Ultimately, 9/11 was the deadliest day for first responders in U.S. history.

Eventually, investigators learned that the terrorists chose to hijack cross-country flights with large fuel loads. The terrorists knew that the fuel would cause destructive fires, which ultimately caused the Twin Towers to collapse and destroyed part of the Pentagon. The terrorists also chose flights with fewer passengers to reduce the chance of resistance.