As the nation mourned the nearly 3,000 people who were killed on 9/11, the George W. Bush administration frantically tried to find its footing and prevent what many feared would be a second wave of attacks.
President Bush ordered members of his administration, including top counterterrorism official Richard Clarke, to imagine what the next attack could look like and take steps to prevent it.
The Bush administration also empowered the FBI and its partners at the CIA, National Security Agency and the Pentagon to take the fight to al-Qaida.
The military invaded Afghanistan, which had been a haven for the group. The CIA hunted down al-Qaida operatives around the world and tortured many of them in secret prisons.
By the early days of the Obama administration, the U.S. had to a large extent hardened the homeland against 9/11-style plots. But the terrorism landscape was evolving.
A few years later, a different terrorist group emerged from the cauldron of Syria and Iraq — the Islamic State, or ISIS, a group that would build on Awlaki’s savvy use of the digital world.