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.