Ceremonies were held in New York City, Virginia and Pennsylvania.
Rather than watching events unfold from afar, Perry Messick answered a call for chaplains to assist at the World Trade Center disaster site in New York City.
A simple chalkboard has become a powerful piece of history. See how firefighters and conservators are working to preserve the names of colleagues lost on Sept. 11.
Victims' families are pressing President Trump to declassify more materials.
Agents across the nation scrambled to prevent attacks on U.S. leadership.
The special "9/11: United We Stand -- 25 Years Later" airs Tuesday, Sept. 1.