US President Joe Biden commemorated the 20th anniversary of the September 11 attacks by visiting each of the sites where hijacked planes crashed in 2001.
Mr Biden and the first lady Jill Biden began the sombre day in New York, attending a ceremony at Ground Zero where the World Trade Centre Twin Towers stood before they were struck by two aircraft.
The site is now a memorial park, a fountain and museum.
Former presidents Barack Obama and Bill Clinton joined Mr Biden, along with their wives, Michelle Obama and former Secretary of State Hillary Clinton, on Saturday.
Mr Biden then travelled to Shanksville, Pennsylvania, where the hijacked United Airlines Flight 93 crashed into a field, after passengers had rushed the cockpit to tackle the hijackers.
It is widely believed that the aircraft had been heading for the White House.
The president took time out to visit the Shanksville Volunteer Fire Department and praised former president George W Bush's speech made earlier in the day.
“These memorials are really important, but they’re also incredibly difficult for the people who were affected by them – because it brings back the moment you got the phone call. It brings back that instant you got the news, no matter how many years go by,” Mr Biden said.
He ended the day with a visit the Pentagon, which was hit by another hijacked passenger jet.
Mr Biden was joined by Vice President Kamala Harris and her husband Douglas Emhoff, where they laid wreaths of remembrance.
After the attacks, the longest war in US history was launched in Afghanistan, where militant group Al Qaeda that carried out the 9/11 attacks had been harboured.
The US pulled out of Afghanistan last month.
Mr Biden released a video on Friday to express his condolences to the loved ones of the victims and highlight the national unity that followed 9/11.
“It's so hard. Whether it's the first year or the 20th, children have grown up without parents and parents have suffered without children,” Biden said.
The president noted the heroism that was seen in the days following the attacks.
“We also saw something all too rare: a true sense of national unity,” Mr Biden said.
Mr Biden pledged to build up such unity after he came into office earlier this year, but the country remains deeply divided.
US presidents often travel to one of the three attack sites on the 9/11 anniversary but it is unusual to go to all three on the same day.
“The president felt it was important to visit each of these three sites to commemorate the lives lost, the sacrifices made on a day that has impacted millions of people across the country but certainly many people in those communities,” White House spokeswoman Jen Psaki said on Friday.
Mr Biden, having lost three immediate family members in tragic circumstances, is often comfortable in the role of consoler-in-chief, even giving grieving families his personal phone number.
“No matter how much time has passed, and these commemorations bring everything painfully back, as if you just got the news a few seconds ago. And so on this day, Jill and I hold you close in our hearts and send you our love,” he said.