9/11 services in New York City, DC, Pennsylvania

They also sang "God Bless America".

A girl touches the National September 11 Memorial, ahead of the 15th anniversary of the attacks in Manhattan, New York, local time September 9, 2016. The terrorists took control of the aircraft, crashed the Twin Towers of the World Trade Center in New York City, killing everyone on board and many others working in the buildings. As we watched planes fly into buildings, as the magnitude of the loss enveloped us, as we learned the field in Pennsylvania was evidence of an unimaginable sacrifice those fearless souls made, our tears were a sign of the love and compassion we had for our fellow Americans.

A girl, about 6 years old, in a blue-and-white striped dress, held a single yellow chrysanthemum and followed her family through the memorial garden to place the flower on one of the 184 curved benches, a narrow channel of rippling water reflecting light below them, spaced between crape myrtle trees. The names of the victims will be read and bells ring in their memory.

One of the hijacked planes crashed outside Shanksville, Pennsylvania, apparently forced down short of the terrorists' intended target after passengers and crew fought back.

This story has been corrected to show Pennsylvania center opened last year, not this year.

For those not already tuned in and unable to look away from the television, there were phone calls telling us to turn on the news, the shock and horror of the video and photos of the attack and the collapse of the Twin Towers, efforts to reach friends and loved ones feared to be in harm's way, and trying to make sense of what had happened.

Clinton spokesman Nick Merrill said in a statement Sunday that the former secretary of state attended the morning ceremony for 90 minutes "to pay her respects and greet some of the families of the fallen". Neither candidate is expected to make public remarks.

The new PATH terminal in the World Trade Center at night, viewed from across the 9/11 memorial plaza.

Trump - a NY real estate mogul - has said he donated construction equipment to the recovery effort and gave $100,000 to the memorial after touring it for the first time earlier this year.

But just as Pearl Harbor was a much more pivotal moment for the Greatest Generation than for their children and grandchildren, 9/11 has begun to fade somewhat from our collective consciousness.

Close to 3,000 people died when planes crashed into the towers, as well as the Pentagon in Washington DC and a field in Shanksville, Pennsylvania.

The attacks caused a total of 2,996 deaths, making it the deadliest foreign attack ever on USA soil.

"The most enduring memorial.is ensuring the America we continue to be, that we stay true to ourselves, stay true to what is best in us, that we not let others divide us", Mr Obama said.

The 15 anniversary of the attacks falls in the midst of the heated presidential elections.

Bagpipes played and bells tolled in solemn tributes.

Another hijacked plane hit the Pentagon, the headquarters of the U.S. Defense Department.

The United States on Sunday commemorated the 15th anniversary of the 9/11 attacks with a moment of silence observed in somber remembrance at Ground Zero in NY, where almost 3,000 people were killed.

Organizers included some additional music and readings Sunday to mark the milestone year.

Each year, family members spend hours reading out the names of all the dead at the memorial, an increasing number of them young adults who never or barely knew lost parents, uncles, aunts and grandparents.

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