Ceremonies were held in New York City, Virginia and Pennsylvania.
The anniversary of the 9-11 attacks is now a federally recognized National Day of Service.
Twenty-five years after the Sept. 11 attacks, New York City marked the anniversary with a day of remembrance, moments of silence and heightened security in Lower Manhattan.
Among the people who have helped keep the Tribute in Light shining is Valerie Webb, who was asked to flip the switch for the first Tribute in Light just six months after the attacks.
More than 9,400 people have died from 9/11-related illnesses.
A story of family, sacrifice and legacy. The triplet sons of a firefighter who responded on Sept. 11 are finding their own ways to serve.