Deep inside the museum at the 9/11 Memorial in downtown Manhattan, on a blue-tiled wall, is inscribed a phrase from the ancient Roman poet Virgil: "No day shall erase you from the memory of time."

Yet, 20 years after two hijacked commercial aircraft slammed into the soaring Twin Towers on that site that September morning in 2001, killing almost 3,000 - including in other attacks, one of which failed to hit its target and the other striking the Pentagon near Washington - the trauma of that day itself has perhaps inevitably receded.

Certainly the event is always, and will continue to be, marked in sombre remembrance. And certainly an overwhelming number of Americans who are old enough to recall the day remember where they were, and what they were doing, when they heard of the attacks.

But a growing number have no personal memory of the day, either because they were too young then, or not born yet, Pew Research said this month.

"9/11 transformed US public opinion, but many of its impacts were short-lived," said Pew.

Patriotic sentiment surged in the aftermath of 9/11 as the US invaded Afghanistan. But so did fear and worry. In the days and weeks after, most Americans said they were praying more often, Pew said. Xenophobia resurfaced and "Islamophobia" became a phenomenon.

The federal government created the Department of Homeland Security in 2002 - now the third-largest Cabinet department.

Security was tightened across the board, especially after Richard Reid, a man reportedly radicalised in Pakistan and Afghanistan, tried to blow up a plane with explosives planted in his shoes - the reason why commercial airports in the US require passengers to take off their shoes for scanning at security gates.

"9/11 destroyed the sense of geography - the safety of two oceans as a protector from the nasty world out there - and created a new sense of vulnerability," says Dr Robert Manning, a senior fellow at the Atlantic Council's Scowcroft Centre for Strategy and Security. "It was a big psychological shift."

Public support for the war in Afghanistan remained positive for a number of years. Trust in government was also high.

But both of those as well as concerns about terrorism have dwindled in recent years. "Sept 11... has become like virtually all American holidays," says Dr Glenn Altschuler, professor of American Studies at Cornell University.

"The occasion is marked, people know what it's for, but it no longer has the urgency. In the 20 years since 9/11, because there has been no significant international terrorist event on American soil, Americans no longer have the apprehensiveness, concern, attention that it commanded in its early days.

"There have been various attempts by various people to stir up fears of terrorists coming in through Mexico or Muslim immigrants or that sort of thing. But beyond a relatively small sliver of the population, American concerns do not place terrorism at or anywhere near the top."

Pew's surveys reveal that "the share of Americans who point to terrorism as a major national problem has declined sharply as issues such as the economy, the Covid-19 pandemic and racism have emerged as more pressing problems in the public's eyes".

"In 2016, about half of the respondents (53 per cent) said terrorism was a very big national problem," Pew said. "This declined to about four in 10 from 2017 to 2019."

By last year, only a quarter of Americans said terrorism was a very big problem. The Federal Bureau of Investigation, in fact, now says that domestic terrorism is a greater threat to the homeland than international terror.

The psychological shock of 9/11 has faded, says Stanford University professor Larry Diamond.

"Particularly now, in the midst of Covid-19, and with the withdrawal from Afghanistan reflecting exhaustion with the burdens of being there; and with the crisis in our democracy, and all of the economic difficulties; and on the foreign policy front, the rise of China and the existential threat it poses both in economic and security terms, concern about Islamic radical terrorism is very far from being front and centre.

"I think we have vastly better intelligence than we used to have. So there were enduring changes that Americans don't think about and question."

He added: "But I've got to tell you, in the midst of everything we're dealing with, I don't think the anniversary of 9/11 is actually resonating very deeply with the American public."

New Yorker Praveen Vajpeyi, who was 38 in 2001, was one of the many thousands commuting to work on that fateful September morning. He popped out of the subway into a silent crowd watching with horror the tragedy unfolding in downtown Manhattan.

They saw, heard and felt the collapse of the South Tower at 9.59am. It created a mini earthquake logged about 34km away at the Columbia Observatory, registering 2.1 on the Richter scale. Twenty-nine minutes later, he saw the second tower collapse from Madison Square Park.

Reflecting on that day, Mr Vajpeyi says: "It is very difficult for the families who lost people, but what is also true is that every year we make a spectacle of it to some extent."

Also, the fact that the US lashed out and tens of thousands - close to 50,000 Afghan civilians, and well over 184,000 Iraqi civilians in the subsequent war on Iraq - were killed does not figure in the broader narrative.

"What happened after 9/11 needs to be mourned as well," Mr Vajpeyi said.

But there is also no doubting the resilience of the city of New York, or of the United States.

In the two large black squares at the Memorial, located in the exact footprints of the destroyed Twin Towers, surrounded by oak trees, water flows down the strangely serene yet also unforgiving black walls, inward towards the centre where it disappears into a smaller, ominous, abyss-like square.

Above it all, overlooking the names inscribed on the bronze parapets below, soars the tallest building in the US - the 104-floor One World Trade Center, which opened in 2014.