Jose Jimenez/Primera Hora/Getty ImagesPolicemen and firemen run away from the huge dust cloud caused as the World Trade Center's Tower One collapses after terrorists crashed two hijacked planes into the twin towers, September 11, 2001 in New York City.
For others, it was an occasion to urge for a return to the sense of unity they felt after the atrocities.
But each ceremony has personal touches, too.
Trump also plans to participate in a 9/11 observance at the Pentagon.
President Trump will lead a moment of silence at the Pentagon at 9:37 a.m. ET, the moment American Airlines Flight 77 struck the building.
In Shanksville, Pennsylvania, Vice-President Mike Pence addressed hundreds of people, including relatives of victims of United Flight 93, at a sombre service.
Additionally, the names of each victim will be read at the National September 11 Memorial and Museum.
The president and first lady Melania Trump joined with Defense Secretary Jim Mattis, members of his Cabinet and military personnel at the Pentagon to observe the anniversary of the attacks on the nation's defense headquarters.
"It does feel good to know you have other people who are feeling the same pain that you're in", Marvaline Monroe said as she headed into the ceremony to remember her brother, Keith Broomfield. A JNF monument in Arazim Valley near Jerusalem, called the 9/11 Living Memorial, is the only monument outside of the United States that lists the names of all the victims.
GettyThe al-Qaeda terrorists who carried out the attacks.