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 attacks orchestrated by al-Qaida terrorists.
In New York, families and friends of victims gathered at 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.
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.
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.