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.
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 later took part in a ceremony at the Pentagon for family members of the more than 100 people killed when a jetliner struck the U.S. military headquarters.
“We pray for the memory of all those taken from us, nearly 3,000 innocent souls. Our hearts still ache for the futures snatched away,” the president said.
Another 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.