New York: America commemorated the 15th anniversary of the 9/11 attacks on Sunday with emotional services of remembrance in New York, Washington and Pennsylvania to honour those who perished in the world's deadliest terror strikes.
On 11 September, 2001, 19 Al-Qaeda operatives crashed four passenger jets into the Twin Towers in Manhattan, the Pentagon and a field in Shanksville, Pennsylvania — killing nearly 3,000 people and changing the world forever.
This year's anniversary comes with the United States locked in a bruising White House election battle between Hillary Clinton and Donald Trump, who both attended the New York service, although Clinton left early after feeling unwell.
Even 15 years later, the long shadow cast by the attacks lives on in wars being fought today in Iraq and Afghanistan, and conflict tearing apart countries from Libya to Syria, allowing Al-Qaeda affiliates and the Islamic State (IS) terror group to breed and prosper.
President Barack Obama said no words or deeds could ever truly erase the pain of loss, but urged Americans to stand true to the nation's ideals and not allow groups like Al-Qaeda and IS to divide the country.
"It is so important today that we reaffirm our character as a nation," he told a remembrance service at the Pentagon.
"Our diversity, our patchwork heritage, is not a weakness. It is still and always will be one of our greatest strengths."
In New York, relatives fought back tears, clasped onto each other and bowed their heads at the 11 September Memorial on the site of the destroyed World Trade Center, which was closed to the general public.
The emotional service — in the shadow of the newly built Freedom Tower — observed six moments of silence to honour the four attacks and the moments each of the Twin Towers collapsed.
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.
Mourners sobbed and laid flowers in the grooves of their loved ones' names, carved into the walls of two reflecting pools in the footprint of the towers overlooked by the Freedom Tower, the tallest building in the Western Hemisphere.
The US government says the country is now better protected against a 9/11-style terror attack, but the new threat is the lone-wolf assailant.
"Our government has become pretty good at detecting something hatched from overseas," Homeland Secretary Jeh Johnson told Fox News.
"Where we're challenged, however, is with the lone-wolf style attack, the self-radicalized actor. Terrorist organisations have the ability to (get) into our homeland through the internet and recruit and inspire."
The United States, but more increasingly Europe, have been hit by such attacks, including the 2013 Boston Marathon bombings and the 2015 San Bernardino killings in California.
"We'll never forget the horror of September 11, 2001. Today, let's honor the lives and tremendous spirit of the victims and responders," tweeted Clinton, who was a US senator from New York at the time of the attacks.
Trump called the anniversary "a day of sadness and remembrance" but also "a day of resolve."
It was the country's "solemn duty," he said in a statement, "to work together as one nation to keep all of our people safe from an enemy that seeks nothing less than to destroy our way of life."
George W Bush, who was president at the time of the attacks, spent the morning at church in Dallas, Texas, his home state.
He was due to attend the Dallas Cowboys home opener against the New York Giants, where he will take part in the ceremonial coin toss with two New York police officers who were at Ground Zero on 9/11.
Sunday marks the start of the NFL season in the United States, and those attending the American football games and watching on television will watch video messages from both Obama and Bush.