NEW YORK — The nation’s gathering war against a new upsurge in Islamic terror hung heavy over the 13th anniversary of the Sept. 11 attacks Thursday, stirring both anxiety and determination among those who came 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 authorizing stepped-up airstrikes 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 Poptean. “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 1,776-foot One World Trade Center.

The attacks were also commemorated in Shanksville, Pennsylvania, where former House Speaker Dennis Hastert gave the flag that flew atop the U.S. Capitol on 9/11 to the Flight 93 National Memorial.

At the Pentagon, where Obama spoke at a wreath-laying ceremony, he didn’t 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.”

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 9/11 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 U.S. 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 Lowe, died at the World Trade Center.

While little about the annual ceremony at ground zero has changed, much around it has.

When the underground National Sept. 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.

On Thursday 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 Center.

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.”

Bronx resident Craig Bunnell, 23, who said he attends college in the area, thought it was important that he be there. “I figured of all days, I should stop by and pay respects,” he said.

Still, “coming down to the area is rough,” said Franklin Murray, who wore a shirt with a photo of his slain brother, Harry Glenn, to Thursday’s ceremony.

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 Burlingame was the pilot of the hijacked plane that crashed into the Pentagon.

Others said they fear the tragedy that took place in the neighborhood is being forgotten.

“Instead of a quiet place of reflection, it’s where kids are running around,” said Nancy Nee, who lost her firefighter brother, George Cain. “Some people forget this is a cemetery. I would never go to the Holocaust museum and take a selfie.”

Around the country, observances were held in such places as Morrison, Colorado, where hundreds of people walked the equivalent of the twin towers’ 110 stories by going up and down stairs at the Red Rocks Amphitheatre, and Point Lookout, New York, where two 18-foot, sand-covered towers were crafted in remembrance.

In New York City, some relatives who read the long list of names touched on the attacks’ legacy. In one family, two boys are named for an uncle they never met, financial worker Michael Wittenstein. In another family, 17-year-old Jordan Thompson joined the Marines in memory of his uncle, Leon Bernard Heyward, a city consumer affairs worker.

“In your honor,” Thompson said, “I have decided to serve our country.”