Americans are marking the 21st anniversary of 9/11 with tributes to those who died in the deadliest terror attack on US soil.

The commemoration began with a tolling bell and moment of silence at ground zero, the former site of New York’s Twin Towers, which were destroyed after hijacked planes crashed into them on 11 September 2001.

Victims’ relatives and dignitaries also convened at the Pentagon and in a field in Pennsylvania, where other aircraft seized by al Qaeda terrorists went down.

Nearly 3,000 people were killed in total.

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Communities are marking the day with candlelit vigils, interfaith services and other commemorations.

Some joined volunteer projects on a date that is federally recognised as both Patriot Day and a National Day of Service and Remembrance.

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President Joe Biden will speak and lay a wreath at the Pentagon, while First Lady Jill Biden is scheduled to speak in Shanksville, Pennsylvania.

An aircraft crashed in a field there after passengers and crew members tried to storm the cockpit as the hijackers headed for Washington.

Vice President Kamala Harris and her husband will attend the National September 11 Memorial in New York, where victims’ relatives traditionally read aloud the names of the dead.

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Some are forced to remember, others want to, but New York will never forget

‘Every 9/11 is a reminder of what I lost’

The observances follow a fraught milestone anniversary last year, which came weeks after the chaotic and humbling end of the Afghanistan war that the US launched in response to the attacks.

As well as spurring the war on terror, the attack stirred a sense of national pride and unity for many, while subjecting Muslim Americans to years of suspicion and bigotry.

The attacks have cast a long shadow over the personal lives of thousands of people who survived, responded or lost loved ones, friends, and colleagues.

More than 70 of Sekou Siby’s co-workers perished at Windows on the World, the restaurant at the top of the trade centre’s north tower.

Mr Siby, an Ivorian immigrant, was scheduled to work that morning until another cook asked him to switch shifts.

He never took a restaurant job again, and he has wrestled with how to comprehend such horror in a country where he had gone to seek a better life.

“Every 9/11 is a reminder of what I lost that I can never recover,” said Mr Siby.

He is now president and chief executive of restaurant workers’ advocacy group ROC United, which evolved from a relief centre for Windows on the World workers who lost their jobs when the twin towers fell.