Britain's Queen Elizabeth said on Saturday her prayers remain with victims and survivors of the September 11 attacks on the United States in 2001.
She paid tribute on the 20th anniversary of the attacks to the communities that joined together to rebuild after the devastation.
"My thoughts and prayers - and those of my family and the entire nation - remain with the victims, survivors and families affected, as well as the first responders and rescue workers called to duty," she said in a message to US President Joe Biden.
The atrocity saw 19 Al Qaeda extremists hijack four commercial planes to carry out devastating suicide attacks against the US.
Two planes were flown into the Twin Towers of New York's World Trade Centre, which then collapsed, a third plane was flown into the Pentagon just outside Washington, while crew members and passengers on a fourth plane forced hijackers to crash in Pennsylvania.
"My visit to the site of the World Trade Centre in 2010 is held fast in my memory," the Queen said.
"It reminds me that as we honour those from many nations, faiths and backgrounds who lost their lives, we also pay tribute to the resilience and determination of the communities who joined together to rebuild."
The 9/11 attacks killed 2,977 people, the single largest loss of life resulting from a foreign attack on American soil.
Sixty-seven British nationals were among the dead.
Mr Biden is commemorating the 20th anniversary attacks by visiting each of the sites where hijacked planes crashed, seeking to honour the victims.
The anniversary comes shortly after the chaotic end of the US-led war in Afghanistan, launched 20 years ago against Al Qaeda and its leader, Osama bin Laden, who was killed in 2011 by US special forces in Pakistan.