An estimated 3,000 deaths were tallied at the end of the day, 9/11/2001.
There was no question who was behind it. Intelligence sources twice briefed President George W. Bush about it. That night, the United States was at war with al-Qaida.
The Pentagon issued a news release Wednesday, 11 years later, quoting an official saying that al-Qaida has been crushed. The U.S. has not experienced a large-scale terrorist attack since 9/11.
But one man in the president's bunker on 9/11 said the fight is not over.
Now-retired Lt. Col. Robert J. Darling was with Vice President Dick Cheney and Secretary of State Condoleeza Rice while an airplane flew over the White House to crash into the Pentagon and spoke with Russian Prime Minister Vladimir Putin to explain why the president had authorized an increase in the nation's defense readiness.
"There is still Hamas, Hezbollah and other spin-off groups that al-Qaida can morph into," Darling said Thursday in an interview prior to his presentation on 9/11 at Penn State Altoona.
Darling's daily job as the Marine Corps representative to the White House Military Office was solely to coordinate the transport of presidential equipment: Secret Service vehicles, phones with connection to the White House and naval fleets traveling under Air Force One, in advance of the president.
That was his job on 9/11 when Bush, pushing his education campaign in Florida, was told a second plane struck the south tower of the World Trade Center in New York.
Darling was the link between the Situation Room and Vice President Dick Cheney during 9/11.
He was the one person not to evacuate the White House at 9:45 a.m just after a plane had struck the Pentagon. Instead, he worked from the president's bunker, the exact location of which is the only thing Darling won't talk about from that day.
He said his goal was to urge people to appreciate the desperate decisions Bush and Cheney made on 9/11, and to inspire students to engineer quick-acting vaccines and technology that will protect future generations of Americans.
During a discussion after Darling's presentation, one audience member said he was on the verge of tears when he learned of Cheney's decision to authorize Flight 93, hijacked and headed south of Pittsburgh to Washington, to be shot down.
"I don't think I would have been able to do it," he said.
Cheney made that decision because communication was dropped between Bush, who was leading the country from Air Force One, and the Secretary of Defense. That lapse of communication must never happen again, Darling said. But in that case, orders from Cheney were valid.
Flight 93 crashed in Shanksville, not because of Cheney's orders to shoot it down, but because those on board formulated a plan and rolled the plane upside down.
"If that isn't heroism, then I don't know what is. ... I tell this story so that we don't forget that our leaders were willing to take American lives to save American lives. I don't ever want to see that happen again," Darling said.