Today marks the 14th anniversary of the 9/11 terror attacks in New York City.
The largest memorial ceremony will take place at the National September 11 Memorial and Museum near the site of the World Trade Center's twin towers.
The attack, which saw two hijacked passenger jets crash into the Twin Towers, killed nearly 3,000 people and left thousands more injured.
Families of the victims and invited guests will gather at the city's memorial plaza for the ceremony which will feature a moment's silence and a reading of the names of those who died.
The plaza, which saw an estimated 20,000 visitors to the site on this day last year, will open to visitors later in the morning.
At the Pentagon, the newly-opened Flight 93 National Memorial will see officials attend a commemoration to the victims.
Meanwhile, thousands are paying tribute to the victims online as the hashtags #September11 and #NeverForget911 trend on Twitter.
14 years since 9/11 #wherewereyou— Gareth (@garethoconnor) September 11, 2015