Memorials were held in New York City, Washington D.C. and Pennsylvania on Tuesday to mark the 17th anniversary of the Sept. 11 attacks.
In New York, the ceremony took place at the National September 11 Memorial and Museum, located where the World Trade Center towers once stood.
The ceremony started at 8:39 a.m., and the reading of names proceeded with six pauses of silence, each marking a time when the planes hit the two towers and the Pentagon, when another crashed in Pennsylvania, and when the towers crashed.
The ceremony is scheduled to last until 12:30 p.m.
At the U.S. Department of Defense near Washington D.C., a ceremony featuring U.S. Vice President Mike Pence and Defense Secretary James Mattis began at sunrise with a U.S. flag unfurling on one side of the Pentagon building.
The names of the 184 people who died here were read, followed by remarks by Mattis and Pence.
U.S. President Donald Trump visited Shanksville, Pennsylvania, where United Airlines Flight 93 crashed, killing 40 on board.
Hundreds of people turned out for the event despite cloudy and chilly weather with a light drizzle, to hear the names of those 40 called along with bell tolls.
Trump hailed the heroism the passengers on flight 93 demonstrated when they fought for control of the aircraft after learning the hijackers' intentions, telling their relatives "America will never forget what your loved ones did for all of us."
The United States came under multiple terrorist attacks on Sept. 11, 2001, after four civilian aircraft were hijacked and steered toward prominent U.S. buildings. Three of the planes hit their targets in New York and Washington D.C., while the fourth crashed en route to Washington as its passengers battled for control of the aircraft with the hijackers.
The attacks left nearly 3,000 killed or missing and were the largest terrorist attacks on U.S. soil in history.