Former President Barack Obama (left), Michelle Obama, President Joe Biden, first lady Jill Biden and former New York City Mayor Michael Bloomberg attend the annual 9/11 commemoration ceremony at the National September 11 Memorial & Museum in New York on Saturday. AP

America marked the 20th anniversary of 9/11 Saturday with pleas for unity at solemn ceremonies as the Federal Bureau of Investigation released the first document related to its investigation of the 9/11 attacks on the United States, following an executive order by President Joe Biden, CNN reported.

Biden commemorated the 20th anniversary of the Sept. 11 attacks on the United States on Saturday with visits to each of the sites where hijacked planes crashed in 2001, honouring the victims of the devastating assault.

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At the 9/11 memorial in New York, relatives wiped away tears, their voices breaking as they read out the names of the almost 3,000 people killed in the Al-Qaeda attacks, the deadliest in history.

Joe Biden (right) and Kamala Harris (2^nd left) participate in a wreath-laying ceremony in Arlington, Virginia. AFP

"We love you and we miss you," they said as sombre violin music played at the official ceremony, attended by President Joe Biden and former presidents Barack Obama and Bill Clinton.

In the first of his wordless tributes, Biden stood in sombre silence with former Presidents Barack Obama and Bill Clinton at the New York City site where planes brought down the World Trade Center's twin towers.

The service at Ground Zero where 2,753 people died — some of whom jumped to their deaths from the burning towers — took place under tight security, with Lower Manhattan effectively locked down.

The three leaders, flanked by Jill Biden, Michelle Obama and Hillary Clinton, shared a moment of silence with the crowd at 8:46am to mark the time that the first plane hit. Heads bowed, they listened as relatives read the names of those who died.

US President Joe Biden and first lady Jill Biden walk down the wall of names of the victims in Stoystown. Reuters

At 9:03 am, attendees stood still again to mark the moment the South Tower was struck. At 9:37 am, it was the Pentagon, where the hijacked airliner killed 184 people in the plane and on the ground.

The Bidens then flew to Shanksville, Pennsylvania, before heading back to the Washington area to visit the Pentagon memorial.

Biden did not deliver remarks at any of the sites. He released a video on Friday to express his condolences to the loved ones of the victims and highlight the national unity that resulted, at least initially, after 9/11.