A tolling bell and a moment of silence began the commemoration at ground zero in New York, where the World Trade Center's twin towers were destroyed by the hijacked-plane attacks of Sept. 11, 2001. Victims' relatives and dignitaries also convened at the two other attack sites, the Pentagon and a field in Pennsylvania.
The university previously gave the same deal to families earning less than $65,000 a year, according to a Princeton news release.
Some Americans are joining in volunteer projects on a day that is federally recognized as both Patriot Day and a National Day of Service and Remembrance.
The observances follow a fraught milestone anniversary last year. It came weeks after the chaotic and humbling end of the Afghanistan war that the U.S. launched in response to the attacks.
But if this Sept. 11 may be less of an inflection point, it remains a point for reflection on the attack that killed nearly 3,000 people, spurred a U.S. "war on terror" worldwide and reconfigured national security policy.
It also stirred — for a time — a sense of national pride and unity for many, while subjecting Muslim Americans to years of suspicion and bigotry and engendering debate over the balance between safety and civil liberties.
In ways both subtle and plain, the aftermath of 9/11 ripples through American politics and public life to this day.
He found it difficult to form the type of close, family-like friendships he and his Windows on the World co-workers had shared. It was too painful, he had learned, to become attached to people when "you have no control over what's going to happen to them next."