US President Barack Obama has joined the nation in remembering the nearly 3,000 people who died in the September 11 attacks 15 years ago.
He observed the sombre anniversary with a moment of silence in the Oval Office on Sunday at 8.46 am EDT - the precise moment the attacks began on a sunny day in 2001 - when a hijacked passenger plane slammed into the north tower of New York City's World Trade Centre.
Afterwards, Mr Obama arrived at the Pentagon, where he laid a large wreath at the beginning of a memorial service.
The American flag has been lowered to half-staff at the White House and other federal buildings. Mr Obama has invited governors, interested organisations and individuals to follow suit.
At Ground Zero, hundreds of victims' relatives and dignitaries gathered to hear the reading of the names of the nearly 3,000 people killed under an overcast sky that shrouded the 1,776ft-tall top of One World Trade Centre, the centrepiece of the rebuilt site.
"It doesn't get easier. The grief never goes away. You don't move forward - it always stays with you," said Tom Acquaviva, of Wayne, New Jersey, who lost his son Paul Acquaviva.
Mr Obama was scheduled to speak at an observance at the Pentagon. Hundreds of people also were expected at a ceremony at the Flight 93 National Memorial in Shanksville, Pennsylvania.
The 15th anniversary arrives in a country caught up in a political campaign.
Democratic presidential candidate Hillary Clinton and Republican rival Donald Trump both were at the anniversary ceremony at the World Trade Centre.
Neither candidate was expected to make public remarks at a ceremony where politicians have been allowed to attend, but not speak, since 2011.
Mrs Clinton and Mr Trump also followed a custom of halting television ads for the day.
Later, Mrs Clinton's campaign team said the Democratic presidential nominee had to leave the ceremony after feeling "overheated".
She is "feeling much better", her team said.
Clinton spokesman Nick Merrill said in a statement on Sunday that the former secretary of state attended the morning ceremony for 90 minutes in New York "to pay her respects and greet some of the families of the fallen".
He added: "During the ceremony, she felt overheated so departed to go to her daughter's apartment, and is feeling much better."
The statement offered no additional details, including whether Mrs Clinton required medical attention.
While Ground Zero and the nation around it are forever marked but greatly changed since 9/11, the anniversary ceremony itself has become one of the constants in how America remembers the attacks after 15 years.
Organisers planned some additional music and readings on Sunday to mark the milestone year. But they were keeping close to what are now traditions: moments of silence and tolling bells, an apolitical atmosphere and the hours-long reading of the names of the dead.
"This idea of physical transformation is so real here," September 11 memorial president Joe Daniels said this week. But on this September 11 itself, "bringing the focus back to why we did all this - which is to honour those that were lost - is something very intentional."
The simple, reverential observance may be the norm now, but city officials fielded about 4,500 suggestions - including a Broadway parade honouring rescue workers and a one-minute blackout of all of Manhattan - while planning the first ceremony in 2002.
Financial and other hurdles delayed the redevelopment of the Trade Centre site early on, but now the 9/11 museum, three of four currently planned skyscrapers, an architecturally adventuresome transportation hub and shopping concourse and other features stand at the site. A design for a long-stalled performing arts centre was unveiled on Thursday.
Around the Trade Centre, lower Manhattan now has dozens of new hotels and eateries, 60,000 more residents and ever-more visitors than before 9/11.
