Solemn memorials are being held in New York, Washington and Pennsylvania to honor the victims of the September 11, 2001 terror attacks.
Nearly 3,000 people were killed in the jetliner attacks orchestrated by al-Qaida terrorists.
In New York, families and friends of victims gathered at a memorial near the site where hijacked planes hit the twin towers of the World Trade Center.
In what has become a tradition, the names of the victims were read.
"Jon Leslie Albert, David D. Alger, Jacquelyn Delaine Aldridge Fredrick, Edward Allegretto, Ernest Alikakos, Joseph Ryan Allen, Eric Allen, Richard L. Allen, Richard Dennis Allen and my son, our son, Port Authority police officer Christopher Charles Amoroso. Chris, keep your watchful eye and steady hand on your wife, beautiful daughter Sophia, your brothers, sisters, grandmas, nieces, nephews, family and friends and of course your mama and I. We all love you and miss you very much."
Some people held up photos of loved ones who perished.
In Washington, U.S. President Barack Obama and other dignitaries observed a moment of silence at the time the first hijacked jetliner crashed in New York.
The president is also attending a ceremony at the Pentagon for family members of the more than 100 people killed when a jetliner struck the U.S. military headquarters.
A fourth observance is being held in Shanksville, Pennsylvania, where United Airlines Flight 93 crashed, killing 33 passengers and seven crew.
The plane crashed as passengers attempted to regain control from hijackers, who were believed to be flying toward Washington.
Relatives of the victims joined the National Park Service Tuesday in a groundbreaking ceremony for a visitor center at the Flight 93 Memorial.
Wednesday also is the first anniversary of the terrorist attack on the U.S. consulate in Benghazi, Libya. Ambassador Christopher Stevens and three other Americans were killed.