Leading his first commemoration of the solemn 9/11 anniversary, US President Donald Trump has told mourners that "the living, breathing soul of America [weeps] 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" adding that those who try are destined to join a long list of vanquished enemies "who dared to test our mettle".
The President and first lady Melania Trump observed a moment of silence at the White House on Monday at the exact moment that a hijacked airplane was slammed into the World Trade Centre.
Meanwhile in New York, some 1,000 family members, survivors, rescuers, and officials gathered at a ground zero ceremony at the World Trade Centre that began with a moment of silence and tolling bells.
Then, relatives began reading out the names of the nearly 3,000 people killed when terrorist-piloted planes hit the trade centre towers, the Pentagon, and a Pennsylvania field, hurling America into a new consciousness of the threat of global terrorism.
Some said they could not believe 16 years had passed since a tragedy that "still feels like yesterday".
Mr Trump, a native New Yorker who was in the city on 9/11, told the Pentagon crowd that the attack was worse than the Japanese assault on Pearl Harbor during World War II 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."
Mr Trump also offered words of comfort the many whose loved ones perished in the attacks.
"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," Mr Trump said.
"Today, our entire nation grieves with you."
But 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 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.