Final answer:
The number of people killed 3000, the casualties are horrific.
Step-by-step explanation:
September 11, 2001 Terrorist Attacks
The incident that made it clear that terrorist networks were operating in the United States occurred on September 11, 2001.
On this day, terrorists associated with the extremist group Al-Qaeda hijacked four commercial airplanes.
Two of the planes were flown into the Twin Towers of the World Trade Center in New York City, causing the towers to collapse.
A third plane struck the Pentagon in Washington, D.C., and the fourth, which was headed towards another target in Washington, D.C., crashed in Pennsylvania after passengers attempted to regain control of the aircraft.
The attacks resulted in the deaths of nearly 3,000 people and were the impetus for the United States' War on Terror, which included military campaigns in Afghanistan and Iraq.
This tragic event not only highlighted the presence of terrorist networks within the U.S. but also set in motion fundamental questions about security, privacy, and the impact of terrorism on national and global politics.
In response to these attacks, the U.S. implemented stringent security measures and engaged in an extensive international military campaign to dismantle terrorist networks and prevent future attacks.