Americans on Wednesday marked the 12th anniversary of the September 11, 2001 attacks in somber fashion, with ceremonies of remembrance in New York and Washington.
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 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.