The nation's gathering war against a new upsurge in Islamic terror has hung heavy over the 13th anniversary of the September 11 attacks, stirring both anxiety and determination among those who went to ground zero to remember their loved ones.
The familiar silence to mark the attacks and the solemn roll call of the nearly 3,000 dead came just hours after President Barack Obama told the country he is authorising stepped-up air strikes in Iraq and Syria against Islamic State extremists.
"It's an ongoing war against terrorists. Old ones die out and new ones pop up," Vasile Poptean said as he left the ceremony, where he had gone to remember his brother Joshua. "If we don't engage them now, there's a possibility there will be another 9/11 down the road."
Victims' relatives and dignitaries gathered in the plaza where the twin towers once stood, an area of shimmering new skyscrapers, including the soon-to-open 1,776ft (541m) One World Trade Centre.
The attacks were also commemorated in Shanksville, Pennsylvania, where former House speaker Dennis Hastert gave the flag that flew atop the US Capitol on September 11 2001 to the Flight 93 National Memorial.
At the Pentagon, where Mr Obama spoke at a wreath-laying ceremony, he did not mention the rise of Islamic State extremists specifically but noted: "We cannot erase every trace of evil from the world."
"That was the case before 9/11," the president said, "and that remains true today."
Mr Obama's nationally televised announcement of his plans to "degrade and ultimately destroy" the militants, coming on the eve of the anniversary, sparked mixed feelings among September 11 victims' relatives. Some saw it as a sign of determination, others as bad timing.
"We're all walking out the door today with tragic and sad and scary memories on us. ... It's an invitation to fight on a day where we lost," said Ellen Mora, who lost her cousin, Robert Higley. But she noted that her mother felt differently, seeing the speech as "us standing tall on the anniversary".
So did Tom Langer, who lost his pregnant sister-in-law, Vanessa Langer. "Thirteen years later, it feels like the world is still paying attention," he said.
Still others lamented that the US was still battling terrorists 13 years after the attacks.
"We're fighting for nothing. We lost so many already, and we will lose so many more," said Gary Lanham, whose father Michael died at the World Trade Centre.
While little about the annual ceremony at ground zero has changed, much around it has.
When the underground National September 11 Memorial Museum opened this spring, fences around the memorial plaza above it came down, making it more easily accessible to visitors and passers-through.
Yesterday evening, crowds of people gathered around the reflecting pools, where the names of the dead are etched. Some took photos of the buildings, including an almost finished One World Trade Centre.
Diane Hartel of Chicago, in the city on a business trip, said the plaza being open to the public "lets other people share in what has happened here".
Some victims' family members view the growing sense of normalcy around ground zero as a sign of healing.
"I want to see it bustling," said Debra Burlingame, whose brother Charles was the pilot of the hijacked plane that crashed into the Pentagon.