India, Sept. 11 -- Twenty-five years after the September 11, 2001, terrorist attacks, their impact continues to be felt across the United States. The attacks not only claimed nearly 3,000 lives but also transformed aviation security, intelligence operations, immigration policy, military strategy and counterterrorism.

Nineteen al-Qaeda members hijacked four commercial aircraft that morning. Two planes crashed into the World Trade Center in New York, another struck the Pentagon, while passengers aboard the fourth aircraft attempted to overpower the hijackers before it crashed in Pennsylvania.

9/11 Attacks Timeline: What Happened That Morning?

The attacks unfolded over less than two hours but permanently altered the course of US history....