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Rivals for the White House have displayed a moment of unity as the US marks the 19th anniversary of the 11 September 2001 terrorist attacks.

Democratic presidential nominee Joe Biden greeted Vice-President Mike Pence at the Ground Zero memorial in New York, where each attended ceremonies.

Mr Biden then travelled to Shanksville, Pennsylvania - the site of the Flight 93 memorial - to pay respects.

President Donald Trump spoke there earlier on Friday.

However Mr Biden and Mr Trump did not cross paths because their schedules did not overlap.

Nearly 3,000 people died when four hijacked airliners were crashed into the World Trade Center in New York, the Pentagon and - after passengers fought back - the field in Shanksville, on what is the worst terrorist attack in US history.

How is President Trump marking the anniversary?

Joined by First Lady Melania Trump, Mr Trump attended a morning service at the Shanksville memorial, where Flight 93 crashed after its 40 passengers and crew prevented al-Qaeda hijackers from reaching the US Capitol building.

"To the family members of Flight 93: today every heartbeat in America is wedded to yours," Mr Trump said. "Your pain and anguish is the shared grief of our whole nation."

"The memory of your treasured loved ones will inspire America for all time to come. The heroes of Flight 93 are an everlasting reminder that no matter the danger, no matter the threat, no matter the odds, America will always rise up, stand tall and fight back."

Although both Mr Trump and Mr Biden will refrain from overt campaigning, their separate visits to Pennsylvania are being watched closely, as the state is seen as a key battleground in the 3 November election.

Before his Shanksville visit, Mr Trump made a brief stop in Johnstown, Pennsylvania, to greet members of the state Army National Guard. The iron-and-steel town is in Cambria County - the heart of what has been known as 'Trump country' - in the southwest of the state.

Mr Trump spoke with three officers about local military and thanked them for their service, local media reported.

Mr Trump narrowly beat Hillary Clinton in the state in 2016 and with the election less than eight weeks away, the president is fighting hard to keep towns like Johnstown on his side, says BBC's Tara McKelvey.

Where is Biden?

Mr Biden and his wife, Jill, attended a memorial service and commemoration ceremony at the 9/11 museum in New York City.

They travelled to Shanksville in the afternoon to visit the same memorial as Mr Trump. Mr Biden was seen greeting family members of those killed in the attack.

He and his wife laid a wreath on the memorial as well.

Mr Biden told reporters earlier on Friday he would not talk about anything other than 9/11 and that his campaign had withdrawn political adverts from airing on television.

"We took all our advertising down, it's a solemn day, and that's how we're going to keep it," he said.

Mr Biden greeted Mr Pence at the World Trade Center site in New York earlier in the day, and the pair touched elbows in lieu of a handshake to observe protocols aimed at preventing the spread of coronavirus.

Mr Biden referenced the pandemic in a series of tweets later in the day, saying: "This year, we mark the anniversary of 9/11 in the midst of another crisis that compels us to summon the best of the American people in the face of unconscionable, inconceivable loss."

What's the reaction?

Mr Pence delivered short remarks in New York City and read passages from the Bible, saying: "I pray these ancient words will comfort your hearts and ours."

New York Governor Andrew Cuomo, a Democrat, also joined Mr Pence and Mr Biden at the memorial service.

On US Twitter, hashtags relating to 9/11 are trending, with people remembering the day and sharing stories of loved ones lost in the attacks.

Some on social media have also drawn attention to the anti-Muslim sentiment that spread across the country in the wake of 9/11, noting how they were bullied, stereotyped, or policed.

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Hate crimes rose against Muslims after the 2001 attacks, FBI data found.

What happened on 9/11?

The 9/11 attacks were a series of four coordinated attacks on the US by the Islamist terror group al-Qaeda.

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As well as the 3,000 people killed on 11 September 2001, some 400,000 people were injured or exposed to other contaminants in the aftermath of the attacks, according to the US Centers for Disease Control.

Hundreds of first responders and survivors have died in the years since due to related illnesses, like cancer.

Weeks after the attack, the US sent troops to Afghanistan to combat al-Qaeda. The so-called 'War on Terror' has stretched decades and American intervention in the Middle East continues to be a quagmire of US foreign policy.

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