PHOTOS: Ceremonies mark 17th anniversary of 9/11 attacks
Americans were commemorating 9/11 with somber tributes, volunteer projects and a new monument to victims Tuesday, after a year when two attacks demonstrated the enduring threat of terrorism in the nation's biggest city.
A member of the military walks the grounds of the National 9/11 Pentagon Memorial before the start of the September 11th Pentagon Memorial Observance at the Pentagon on the 17th anniversary of the September 11th attacks, Tuesday, Sept. 11, 2018.
Visitors to the Flight 93 National Memorial in Shanksville, Pa., hold a giant Flight 93 flag during a moment of remembrance Monday, Sept. 10, 2018, as the nation marks the 17th anniversary of the Sept. 11, 2001, attacks.
Visitors to the Flight 93 National Memorial in Shanksville, Pa., participate in a sunset memorial service on Monday, Sept. 10, 2018, as the nation marks the 17th anniversary of the Sept. 11, 2001 attacks.
Bianca Burton, center left, and Erin Schultheis walk around the Pepperdine University's annual display of flags honoring the victims of the 9/11 terrorist attacks, Monday, Sept. 10, 2018, in Malibu, Calif. Terrorists used hijacked planes to crash Sept. 11, 2001, into the World Trade Centers, the Pentagon and a field in Pennsylvania. Nearly 3,000 people were killed in the attacks.
A U.S. flag is unfurled at sunrise on Tuesday, Sept. 11, 2018, at the Pentagon on the 17th anniversary of the Sept. 11, 2001, terrorist attacks.
A woman leaves flowers at the North Pool during a ceremony marking the 17th anniversary of the terrorist attacks on the United States on Tuesday, Sept. 11, 2018, in New York.
Audience members attend the September 11th Flight 93 Memorial Service, Tuesday, Sept. 11, 2018, in Shanksville, Pa. President Donald Trump is marking 17 years since the worst terrorist attack on U.S. soil by visiting the Pennsylvania field that became a Sept. 11 memorial.
A woman weeps by herself as she leans against a tree during a ceremony marking the 17th anniversary of the terrorist attacks on the United States. Tuesday, Sept. 11, 2018, at the World Trade Center in New York.
Vice President Mike Pence and Karen Pence during wreath laying ceremony at the September 11th Pentagon Memorial Observance at the Pentagon on the 17th anniversary of the September 11th attacks, Tuesday, Sept. 11, 2018. Looking on are Vice Chairman of the Joint Chiefs General Paul J. Selva, and Defense Secretary James Mattis, far right. (AP Photo/Pablo Martinez Monsivais)
A man talks with a girl as they look at the North Pool at the World Trade Center during a ceremony marking the 17th anniversary of the terrorist attacks on the United States, Tuesday, Sept. 11, 2018, in New York. (AP Photo/Mark Lennihan)
Bill Anders, left, and Sharon Custer ring bells as part of the Name Presentation and Ringing of Bells Remembrance during the September 11th Flight 93 Memorial Service in Shanksville, Pa., attended by President Donald Trump and first lady Melania Trump, Tuesday, Sept. 11, 2018. (AP Photo/Gene J. Puskar)
People look on during the September 11th Flight 93 Memorial Service, Tuesday, Sept. 11, 2018, in Shanksville, Pa. (AP Photo/Evan Vucci)
A man stops to lay flowers on one of the benches at the National 9/11 Pentagon Memorial before the start of the September 11th Pentagon Memorial Observance at the Pentagon on the 17th anniversary of the September 11th attacks, Tuesday, Sept. 11, 2018. (AP Photo/Pablo Martinez Monsivais)
President Donald Trump and first lady Melania Trump, escorted by Stephen Clark, Superintendent of the National Parks of Western Pennsylvania, walk along the September 11th Flight 93 memorial, Tuesday, Sept. 11, 2018, in Shanksville, Pa., escorted by (AP Photo/Evan Vucci)
President Donald Trump and first lady Melania Trump, escorted by Stephen Clark, Superintendent of the National Parks of Western Pennsylvania, walk along the September 11th Flight 93 memorial, Tuesday, Sept. 11, 2018, in Shanksville, Pa. (AP Photo/Evan Vucci)