Twenty years ago, the nation watched transfixed as planes hijacked by terrorists crashed into the World Trade Center in New York City, the Pentagon in Washington, D.C. and a field in Shanksville, Pennsylvania on Sept. 11, 2001. The events of that day forever transformed the country.

Nearly 3,000 people died as a result of the attacks, including the 19 hijackers from Islamic extremist group, al-Qaida. It remains the deadliest foreign attack on U.S. soil.  

The Fire Department of New York also lost 343 firefighters dispatched to rescue survivors from the crumbling World Trade Center towers. Today, more than 13,300 9/11 first responders enrolled in the Center for Disease Control and Prevention's 9/11 World Trade Center Program are being treated for cancer.

The attacks prompted then-President George W. Bush to deploy U.S. troops to Afghanistan to hunt for culpable al-Qaida militants sheltered by another Islamic militant group, the Taliban.