The World Has Changed Since 9/11, And So Has America's Fight Against Terrorism

In the fall of 2001, Aaron Zebley was a 31-year-old FBI agent in New York. He had just transferred to a criminal squad after working counterterrorism cases for years.

His first day in the new job was Sept. 11.

"I was literally cleaning the desk, I was like wiping the desk when Flight 11 hit the North Tower, and it shook our building," he said. "And I was like, what the heck was that? And later that day, I was transferred back to counterterrorism."

It was a natural move for Zebley. He'd spent the previous three years investigating al-Qaida's bombings of U.S. embassies in Kenya and Tanzania. And he became a core member of the FBI team leading the investigation into 9/11.

It quickly became clear that al-Qaida was responsible for the attacks.

The hijackers had trained at the group's camps in Afghanistan. They received money and instructions from its leadership. And ultimately, they were sent to the U.S. to carry out al-Qaida's "planes operation."

As the nation mourned the nearly 3,000 people