Events also took place at the other epicentres of the tragedy, including Ground Zero, where the Twin Towers once stood.
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US President Donald Trump and first lady Melania Trump have attended a memorial service at the Pentagon as part of nationwide events on the 24th anniversary of the 9/11 terrorist attacks that killed almost 3,000 people.
The pair appeared on Thursday morning alongside Pete Hegseth, the defence secretary, and General Dan Caine, the head of the Joint Chiefs of Staff, as the names of the victims who lost their lives at the government site in Virginia were read aloud.
Almost 3,000 people lost their lives when two hijacked planes hit the towers in downtown New York in 2001, causing both to collapse.
Dignitaries, politicians and the bereaved also took part in events at the other epicentres of the Al-Qaeda atrocities — Ground Zero in New York, where the Twin Towers once stood, and the field near Shanksville, Pennsylvania, where Flight 93 crashed after passengers and crew fought off four hijackers.
FBI Director Kash Patel was among those at the ceremony at the World Trade Center site.
The bereaved lifted up photos of their loved ones there during a moment of silence that marked the exact time when the first hijacked plane struck one of the two towers.
Jennifer Nilsen, whose husband Troy was killed in the attack, was at the ceremony wearing a t-shirt emblazoned with his name and face.
"Even 24 years later, it's heart wrenching," she said.
Ronald Bucca, whose firefighter father also died in the tragedy, said the annual memorial was a chance to "educate people on that day, especially the younger generations, and learn from each other how to be resilient and deal with loss and rebuild."
This year's remembrance celebrations, which are usually promoted as a day of national unity, come at a difficult time for the US.
On Wednesday, the conservative activist and close Trump ally Charlie Kirk was killed by a shooter at a university campus in Utah. The killer still remains at large as US authorities attempt to locate them.
Figures from across the political spectrum have denounced the violence.
Speaking at the Pentagon, Trump praised Kirk, saying he would posthumously award him the Presidential Medal of Freedom.