President Donald Trump, leading his first commemoration of the 9/11 anniversary, has said "the living, breathing soul of America wept with grief" for each of the nearly 3,000 lives that were lost on that day 16 years ago.
Addressing an audience at the Pentagon, one of three sites attacked on September 11 2001, Mr Trump used the anniversary to sternly warn terrorists that "America cannot be intimidated".
He said those who try are destined to join a long list of vanquished enemies "who dared to test our mettle".
Mr Trump and first lady Melania Trump observed a moment of silence at the White House at the exact moment that a hijacked plane was slammed into the World Trade Centre.
The Trumps bowed their heads and placed their hands over their hearts as the Taps bugle call rang out across the South Lawn.
ABC NEWS: President Trump and First Lady Melania Trump lay wreath at Pentagon 9/11 memorial site in remembrance of terror victims pic.twitter.com/0bG2nt7dk7— Josh Caplan (@joshdcaplan) September 11, 2017
They were surrounded by White House aides and other administration officials in what has become an annual day of remembrance.
Nearly 3,000 people were killed when hijackers flew commercial planes into New York's World Trade Centre, the Pentagon and a field near Shanksville, Pennsylvania.
Mr Trump, a native New Yorker who was in the city on 9/11, said the attack was worse than the Japanese assault on Pearl Harbour during the Second World War because it targeted civilians.
He vowed that it would never be repeated.
"The terrorists who attacked us thought they could incite fear and weaken our spirit," Mr Trump said later at the Pentagon, where he was joined by defence secretary Jim Mattis and General Joseph Dunford, chairman of the Joint Chiefs of Staff.
"But America cannot be intimidated and those who try will join a long list of vanquished enemies who dared test our mettle."
He said that when America is united, "no force on earth can break us apart".
Mr Trump also offered words of comfort for the many whose loved ones perished in the attacks.
Pres. Trump at 9/11 memorial event: “Our people will thrive, our nation will prevail and the memory of our loved ones will never ever die.” pic.twitter.com/WNCRgau7OL— ABC News (@ABC) September 11, 2017
"For the families with us on this anniversary, we know that not a single day goes by when you don't think about the loved one stolen from your life. Today, our entire nation grieves with you," Mr Trump said.
Later, he said "the living, breathing soul of America wept with grief for every life taken on that day".
Vice President Mike Pence was representing the administration at a ceremony at the 9/11 memorial in Shanksville.
Mr Trump has a chequered history with 9/11.
He frequently uses the attack to praise the city's response but has also made unsubstantiated claims about what he did and saw on that day.
Mr Trump often lauds the bravery of New York police officers, firefighters and other emergency responders who rushed to the Twin Towers to help as an example of the resilience of the city where he made a name for himself.
But he has also criticised former president George W Bush's handling of the attacks, accusing Mr Bush of failing in his duty to keep Americans safe.
Mr Trump has made dubious claims about September 11, particularly saying when talking about Muslims that "thousands of people were cheering" in Jersey City, New Jersey, across the Hudson River from lower Manhattan, as the towers collapsed.
There is no evidence in news archives of mass celebrations there by Muslims.
Mr Trump also said he lost "hundreds of friends" in the attack and that he helped clear rubble afterwards.
He has not provided the names of those he knew who perished in the attack, but has mentioned knowing a Roman Catholic priest who died while serving as a chaplain to the city's fire department.