NEW YORK — Americans were commemorating 9-11 with sombre tributes, volunteer projects and a new monument to victims Tuesday, after a year when two attacks demonstrated the enduring threat of terrorism in the nation’s biggest city.

Margie Miller was among the thousands of 9-11 victims’ relatives, survivors, rescuers and others who gathered on a misty Tuesday morning at the memorial plaza where the World Trade Center’s twin towers once stood. She came to the site from her home in suburban Baldwin, as she does 10 or so times a year, to remember her husband, Joel Miller. Only a few fragments of his remains were recovered.

“To me, he is here. This is my holy place,” his widow said before the ceremony began a moment of silence and tolling bells at 8:46 a.m., the time when the trade centre was hit by the first of two terrorist-piloted planes. Victims’ relatives who had brought signs bearing photos of their loved ones wordlessly held them high.

President Donald Trump and Vice-President Mike Pence headed to the two other places where hijacked planes crashed on Sept. 11, 2001, in the deadliest terror attack on American soil.

The president and first lady Melania Trump flew to Pennsylvania to join an observance at the Sept. 11 memorial in a field near Shanksville, where a new “Tower of Voices” was dedicated Saturday. Pence is attending a ceremony at the Pentagon. Trump, a Republican and native New Yorker, took the occasion of last year’s anniversary to issue a stern warning to extremists that “America cannot be intimidated.”

Nearly 3,000 people died in the attacks on 9-11, when international terrorism hit home in a way it previously hadn’t for many Americans. Sept. 11 still shapes American policy, politics and everyday experiences in places from airports to office buildings, even if it’s less of a constant presence in the public consciousness after 17 years.

A stark reminder came not long after last year’s anniversary: A truck mowed down people, killing eight, on a bike path within a few blocks of the World Trade Center on Halloween.

In December, a would-be suicide bomber set off a pipe bomb in a subway passageway near Times Square, authorities said. They said suspects in both attacks were inspired by the Islamic State extremist group.

The recent attacks in New York, as well as terror attacks elsewhere, were on Miller’s mind as she arrived Tuesday.

“You don’t want to live in fear, but it’s very real,” she said.

Debra Sinodinos, who lost her firefighter cousin Peter Carroll and works near the trade centre, said she tries not to let the recent attacks unnerve her.

“You have to move on,” she said as she headed into the anniversary ceremony with her extended family. “Otherwise, you’d live in fear.”

The 9-11 commemorations are by now familiar rituals, centred on reading the names of the dead. But each year at ground zero, victims’ relatives infuse the ceremony with personal messages of remembrance, inspiration and concern.

For Nicholas Haros Jr., that concern is officials who make comparisons to 9-11 or invoke it for political purposes.

“Stop. Stop,” pleaded Haros, who lost his 76-year-old mother, Frances. “Please stop using the bones and ashes of our loved ones as props in your political theatre. Their lives, sacrifices and deaths are worth so much more. Let’s not trivialize them.”

This year’s anniversary comes as a heated midterm election cycle kicks into high gear. But there have long been some efforts to separate the solemn anniversary from politics.

The group 9-11 Day, which promotes volunteering on an anniversary that was declared a national day of service in 2009, routinely asks candidates not to campaign or run political ads for the day. Organizers of the ground zero ceremony allow politicians to attend, but they’ve been barred since 2011 from reading names or delivering remarks.

The names are read by victims’ loved ones, some of them not yet born when the attacks happened.

“Even though I never met you, I’ll never forget you,” Isabella Del Corral said of her grandfather, Joseph Piskadlo.

Hours after the ceremony, two powerful light beams will soar into the night sky from lower Manhattan in the annual “Tribute in Light.”

Memorials to 9-11 continue to grow at Shanksville, where the Tower of Voices will eventually include a wind chime for each of the 40 people killed there, and ground zero, where work is to begin soon on a pathway honouring rescue and recovery workers.

It will serve as a way to honour those who became sick or died from exposure to toxins released when the Trade Center’s twin towers collapsed. Researchers have documented elevated rates of respiratory ailments, post-traumatic stress disorder and other illnesses among people who spent time in the rubble.

About 38,500 people have applied to a compensation fund, and over $3.9 billion in claims have been approved.

Meanwhile, rebuilding continues. A subway station destroyed on 9-11 finally reopened Saturday. In June, doors opened at the 80-story 3 World Trade Center, one of several rebuilt office towers that have been constructed or planned at the site. A performing arts centre is rising.

However, work was suspended in December on replacing a Greek Orthodox church crushed in the attacks; the project hit financial problems.

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Associated Press writers Stephen Groves and Karen Matthews contributed to this report.

Jennifer Peltz, The Associated Press