A New York City Police Department (NYPD) officer pauses at the edge of the south reflecting pool at the National 9/11 Memorial and Museum during ceremonies marking the 16th anniversary of the attacks in New York, US, September 11, 2017. [Photo/Agencies]

NEW YORK - For 64-year-old New Yorker Gary Gonel, who came to the World Trade Center every year on Sept 11 to remember the deadly terror attack in 2001, his sorrow deepened over the thoughts that the society is becoming more divided and people are still worried that the same tragedy could happen again.

"I come here every year to show families of 9/11 that we did not forget about them. Things in the past couple of years have been different, there are fewer and fewer people here," said Gonel, with an American flag in his left hand.

Streets around the World Trade Center "used to be packed with people on Sept 11," but now there are fewer people coming except tourists and families who lost their loved ones, he said.

Sixteen years have passed since a group of terrorists flew hijacked planes into New York's World Trade Center, the Pentagon and a field near Shanksville in the US state of Pennsylvania, on Sept 11, 2001. Nearly 3,000 people were killed during the attacks.

Addressing a 9/11 anniversary commemoration at the Pentagon, US President Donald Trump said on Monday that "America cannot be intimidated." Trump, who was living in the New York city on Sept 11 in 2001, vowed that such an attack would never be repeated.

Meanwhile, Vice President Mike Pence addressed an observance at the Flight 93 National Memorial near Shanksville, Pennsylvania.

A quiet commemoration was being held inside the World Trade Center site on Monday, with thousands of 9/11 victims' relatives, survivors, rescuers and others gathering together to recite all the names of the dead and hold moments of silence.

Outside the site, where Gonel stood, dozens of tourists were taking photos, pigeons were wondering around -- it was just like another ordinary day in September.

A woman wipes away tears as she holds a photograph of Alex Ciccone, a victim of the September 11, 2001 attacks on the World Trade Center, at the National 9/11 Memorial and Museum during ceremonies marking the 16th anniversary of the attacks in New York, US, September 11, 2017. [Photo/Agencies]

While many Americans commemorated this day with tears and wishes, some were worried that as the memory of the tragedy is fading out, similar disastrous incident could happen to the country again.

"I feel like some people have forgotten," Damron Wilson, a volunteer for the commemoration, told Xinhua.

He worried that the American society has "learnt to relax and move on," especially the younger generation.

For Wilson, his feeling of insecurity has worsened when he found that the nation has never been more divided.

"My mum told me that the country had been united after 9/11, but it is deeply divided now. Even today I can see Donald Trump's supporters clashing with people who are against him," he said.

"And with what happened in Charlottesville, I think domestic terrorism is likely to happen (again)," he added.

Last month, violent clashes erupted in Charlottesville, a historic college town in the US state of Virginia, between white nationalists and counterprotesters, leaving one killed and 19 others wounded.

Born and Raised in New York City, Gonel said that he did not feel safe either, especially amid the growing tensions between the city government and the police department.

Staring at the new World Trade Center complex that opened in 2015, Gonel said he was afraid that the center has become more of a tourist spot than a memorial.

"Americans should come here and show some respect. Once you forget, it may happen again," he said.

A woman holding her dog walks amongst 3,000 of US flags at Pepperdine University to mark the 16th anniversary of the 9/11 terror attack, Sept 10, 2017 in Malibu, California. [Photo/IC]

A couple embraces on the Brooklyn Promenade as the Tribute in Light rises above the lower Manhattan skyline, Sept 10, 2017, in New York. The two blue pillars of light provide a visual reminder of how the Twin Towers, destroyed in the terrorist attacks of Sept 11, 2001, once stood above the city skyline. [Photo/IC]

A family member of one of the victims of the crash of Flight 93, reaches out to touch the name of her son after carrying one of the 40 lanterns to the Wall of Names at the Flight 93 National Memorial on Sept 10, 2017 near Shanksville, Pennsylvania. [Photo/IC]

Family, friends and volunteers carry 40 lanterns, one for each victim of the crash, to the Wall of Names at the Flight 93 National Memorial on Sept 10, 2017 near Shanksville, Pennsylvania. [Photo/IC]