Rivals for the White House displayed a moment of unity as the US marks the 19th anniversary of the 11 September 2001 terrorist attacks on Friday.
Democratic presidential nominee Joe Biden greeted Vice-President Mike Pence at the Ground Zero memorial in New York, where each attended ceremonies.
Mr Biden will travel to Shanksville, Pennsylvania – the site of the Flight 93 memorial – later to deliver remarks.
President Donald Trump spoke there earlier.
Though both 2020 candidates will speak in the same place, their schedules will not overlap, ensuring that Mr Biden and Mr Trump will not cross paths.
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 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.”
Before his Shanksville visit, Mr Trump paid a brief visit to Johnstown, Pennsylvania, to greet members of the state Army National Guard.
Mr Trump spoke with three officers about local military and thanked them for their service, the Tribune-Democrat newspaper reported.
Although both Mr Trump and Mr Biden will refrain from overt campaigning, the visits are being watched with close political scrutiny, as Pennsylvania is seen as a key battleground state to win in the November election. Mr Trump narrowly beat Hillary Clinton in the state in 2016.
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, where attackers in 2001 flew planes they had seized into the World Trade Center.
They will travel to Shanksville later in the afternoon to visit the same memorial as Mr Trump.
Mr Biden told reporters on Friday he would not talk about anything other than 9/11 and that his campaign had withdrawn political adverts from airing on television.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.