People across the United States of America (USA) are observing the 21st anniversary of the September 11, 2001 terrorist attacks on Sunday.?

The coordinated terrorist attacks killed a total of 2,977 people. A total of 19 hijackers took control of four passenger planes and crashed one each into the twin towers of the World Trade Centre in New York City. The third plane was rammed into the Pentagon, the headquarters of the US Department of Defence. The fourth plane was brought down in the fields of Pennsylvania after passengers and crew fought back their hijackers.?

Americans are remembering 9/11 with moments of silence, readings of victims' names, volunteer work and other tributes 21 years after the deadliest terror attack on US soil.

Victims' relatives and dignitaries will convene on Sunday at the places where hijacked planes crashed on Sept. 11, 2001.?

Other communities around the country are marking the day with candlelight vigils, interfaith services and other commemorations. Some Americans are joining in volunteer projects on a day that is federally recognised as both Patriot Day and a National Day of Service and Remembrance.

This is the first anniversary of the 9/11 attacks after the US-led Western forces' withdrawal from Afghanistan last year. The US-led Western world had invaded Afghanistan after 9/11 attacks under War on Terror to hunt the terrorist group Al Qaeda's leadership that planned the attacks. Afghanistan's Taliban regime at the time is believed to have sheltered Al Qaeda and the West therefore invaded the country and overthrew the regime.

However, the West soon got involved in a long insurgency against Taliban and other groups and a nation-building project in Afghanistan, which it wrapped up last year. Even before the West withdrew, the Taliban recaptured Afghan capital Kabul and re-established its regime. Al Qaeda, the group held responsible for 9/11 attacks, is also believed to be re-emerging in Afghanistan.

The observances this year therefore follow a fraught milestone 20th anniversary last year. But if this Sept. 11 may be less of an inflection point, it remains a point for reflection on the attack that killed nearly 3,000 people, spurred a US War on Terror worldwide and reconfigured national security policy.

It also stirred —for a time— a sense of national pride and unity for many, while subjecting Muslim Americans to years of suspicion and bigotry and engendering debate over the balance between safety and civil liberties. In ways both subtle and plain, the aftermath of 9/11 ripples through American politics and public life to this day.

And the attacks have cast a long shadow into 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 atop the trade centre's north tower. Siby had been scheduled to work that morning until another cook asked him to switch shifts.

Siby never took a restaurant job again. It would have brought back too many memories. The Ivorian immigrant wrestled with how to comprehend such horror in a country where he'd come looking for a better life.

He found it difficult to form the type of close, family-like friendships he and his Windows on the World co-workers had shared. It was too painful, he had learned, to become attached to people when "you have no control over what's going to happen to them next".

"Every 9/11 is a reminder of what I lost that I can never recover," says Siby, who is now president and CEO of ROC United. The restaurant workers' advocacy group evolved from a relief centre for Windows on the World workers who lost their jobs when the twin towers fell.

On Sunday, President Joe Biden plans to speak and lay a wreath at the Pentagon, while First Lady Jill Biden is scheduled to speak in Shanksville, Pennsylvania, where one of the hijacked planes went down after passengers and crew members tried to storm the cockpit as the hijackers headed for Washington.?

Vice President Kamala Harris and husband Doug Emhoff are due at the National Sept. 11 Memorial in New York, but by tradition, no political figures speak at the ground zero ceremony. It centres instead on victims' relatives reading aloud the names of the dead.

Readers often add personal remarks that form an alloy of American sentiments about Sept 11 — grief, anger, toughness, appreciation for first responders and the military, appeals to patriotism, hopes for peace, occasional political barbs, and a poignant accounting of the graduations, weddings, births and daily lives that victims have missed.

Some relatives also lament that a nation which came together —to some extent— after the attacks has since splintered apart. So much so that federal law enforcement and intelligence agencies, which were reshaped to focus on international terrorism after 9/11, now see the threat of domestic violent extremism as equally urgent.

Moreover, 21 years later, no one accused of planning the 9/11 attacks has been convicted and sentenced. Their trials continue to be in progress after over two decades, a reminder that there has been no justice to almost 3,000 people killed in the attacks and several thousands more left wounded and affected from the events of that day and those that followed.?

(With AP inputs)