President Barack Obama has praised the diversity of the United States and called on Americans not to be divided by their enemies, as he helped mark the 15th anniversary of the worst terrorist attacks to hit U.S. soil.
Speaking at a Pentagon memorial service outside Washington, Obama said the nation will never forget the those killed in the attacks of September 11, 2001, and he said he was inspired by the resilience of the victims' families.
The 9/11 attacks, in which 19 Al-Qaeda terrorists hijacked four commercial airliners, was the deadliest terror strike on U.S. soil, killing nearly 3,000 people.
The attacks led to the United States invading Afghanistan, and later Iraq. Both countries are still wracked by insurgency, terrorism, and war 15 years later.
At the Ground Zero memorial, near the site of the rebuilt World Trade Center in New York City, the names of the dead were read out, as they were at a site near the Pentagon.
There was also a ceremony attended by hundreds of people in Shanksville, Pennsylvania, where one of the hijacked airliners crashed after passengers overpowered the terrorists onboard.
The two politicians hoping to succeed Obama in the White House --
Republican Donald Trump and Democrat Hillary Clinton -- also attended the commemoration ceremonies at the World Trade Center site.
Clinton left the ceremonies earlier than expected after campaign officials said she felt "overheated." A campaign statement offered no additional details, including whether the 68-year-old Clinton required medical attention.
Trump supporters have tried to make the case that she's physically unfit for the White House, citing a concussion she sustained in December 2012 after fainting. Her doctor attributed the episode to a stomach virus and dehydration.
The two have agreed to refrain from campaigning on the anniversary and are not expected to make public remarks. The two presidential candidates will also not broadcast television ads for the day.
George W. Bush, who was president at the time of the attacks, will not attend the ceremonies in New York or Washington.
Instead, his office said he will go to church in Dallas, Texas, and then attend a football game where he will participate in the ceremonial coin toss with two New York police officers who were present during the 2001 attacks.