(AP) -- The U.S. marked the 15th anniversary of 9/11 on Sunday, with a moment of silence marking the instant when a hijacked plane crashed and revealed that the deadliest terror attack on American soil was underway.

At ground zero, hundreds of victims' relatives and dignitaries gathered to hear the reading the names of the nearly 3,000 people killed under an overcast sky that shrouded the 1,776-foot-tall top of One World Trade Center, the centerpiece 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.

President Barack 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.

Nearly 3,000 people died when hijacked planes slammed into the World Trade Center, the Pentagon and a field near Shanksville on Sept. 11, 2001. It was the deadliest terror attack on American soil.

The 15th anniversary arrives in a country caught up in a combustible political campaign, but the nation tries to put partisan politics on hold on the anniversary.

Democratic presidential candidate Hillary Clinton and Republican rival Donald Trump both were at the anniversary ceremony at the World Trade Center. Neither candidate was expected to make public remarks at a ceremony where politicians have been allowed to attend, but not speak, since 2011.

Clinton and Trump also followed a custom of halting television ads for the day.

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.

Organizers planned some additional music and readings 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 hourslong reading of the names of the dead.

"This idea of physical transformation is so real here," Sept. 11 memorial President Joe Daniels said this week. But on this Sept. 11 itself, "bringing the focus back to why we did all this -- which is to honor 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 honoring 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 Center 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, $250 million performing arts center was unveiled Thursday.

Around the Trade Center, lower Manhattan now has dozens of new hotels and eateries, 60,000 more residents and ever-more visitors than before 9/11.

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Associated Press writer Verena Dobnik contributed to this report. Reach Jennifer Peltz on Twitter @ jennpeltz.