NEW YORK: Americans mark the 11th anniversary of the attacks on September 11, 2001 on Tuesday with relatively low key ceremonies that reflect a gradual dampening of passions around the emotional day.

The main ceremony will be the ritual reading at New York’s Ground Zero of the names of the 2,983 people killed both on 9/11 and in the precursor to those attacks, the 1993 car bombing of the World Trade Center.

Relatives of the dead will take turns to read the names against a backdrop of mournful music.

They will pause for moments of silence marking the moment when four planes hijacked by al Qaeda turned into fireballs, two smashing into the Twin Towers, one into the  Pentagon and one crashing into a Pennsylvania field.

Another two moments of silence will be observed at the times the two towers collapsed, accounting for the vast majority of 9/11's victims.

However, this year New York Mayor Michael Bloomberg and other politicians will not take the podium at Ground Zero, in contrast with last year’s 10th anniversary, when President Barack Obama led a long list of VIP guests.

Obama and his wife Michelle will observe the anniversary with a moment of silence outside the White House, then visit the Pentagon memorial.

Vice President Joe Biden, meanwhile, will travel to Shanksville, Pennsylvania, where United flight 93 crashed after passengers attacked the hijackers and thwarted a worse disaster had it continued to its target.

The White House said Obama had been briefed by “key national security principals on... preparedness and security posture” for the anniversary.

But in keeping with the lower key atmosphere this year, there will apparently be no official suspension of the bitter presidential campaign.

Former president Bill Clinton will be campaigning for Obama and speaking out against Republican Mitt Romney at an event in Miami.

The passage of time appears to have cooled public attention to September 11, particularly after the huge media coverage of the 10th anniversary, which many saw as a suitable moment for allowing commemorations to peak.

On the other hand, US Defense Secretary Leon Panetta warned Americans not to forget the troops who are fighting and dying in Afghanistan, as he paid tribute to the victims of the 9/11 attacks on Monday.

In a visit to a memorial in southwestern Pennsylvania honoring the passengers and crew of United Flight 93 that was hijacked on September 11, 2001, Panetta said the fight against the al Qaeda militants behind the attacks was not over, and that soldiers were still in harm's way.

“I pray that as we remember 9/11, and the terrible things that took place on 9/11, that we will also take the time to remind ourselves of the sacrifices that have been made by those who have fought and died in order to make sure that it not happen again,” Panetta told reporters.

“My concern is that too often we do not express our concern and our attention to those who are fighting and dying for this country. We’re continuing to lose good men and women in battle in Afghanistan,” he said.

He drew a connection between the passengers of Flight 93, who struggled with their hijackers and foiled an apparent attempt by Al-Qaeda to strike Washington, and US troops waging war against Taliban insurgents eleven years later in Afghanistan.

The US soldiers are “putting their lives on the line every day,” he said.

“That kind of sacrifice, that kind of commitment, that kind of dedication, that kind of courage is what makes this country strong.

“And we had damn well better remember that every day.”Panetta’s impassioned plea to honor the more than 2,000 American troops killed in Afghanistan and the roughly 77,000 forces deployed there came amid a US presidential campaign that has barely touched on the conflict or foreign policy.

The killing by American troops of al Qaeda founder Osama bin Laden in May 2011 has helped draw a line under 9/11, as has the opening of the Ground Zero memorial, which opened for last year's ceremonies.

A skyscraper at One World Trade Center is near completion and is again the tallest building in New York, as were the Twin Towers before they fell down.

This year sees the publishing on Tuesday of a book by a former US Navy SEAL who was among the troops that shot dead bin Laden in his Pakistani hideout.

The book describes in gory detail how the special forces killed the fugitive, then radioed back the news, saying it was “for God and country.”

The Pentagon has threatened legal action against the author who uses the penname Mark Owen, but has been outed by the US media as Matt Bissonnette.

Last week, Obama said in a radio address that the United States is “stronger, safer and more respected in the world” since 9/11.

But his Republican opponent has accused Obama of weak leadership during the Arab Spring turmoil and of failing to be tough enough on Iran's government.

In Afghanistan, which once hosted bin Laden, US troops continue to struggle to overcome the Taliban guerrilla army.

Most foreign troops are scheduled to withdraw by the end of 2014, handing over responsibility for combat to Western-backed Afghan government forces.

Although there will be no overt campaigning, in such a tight race the two men and their aides always have one eye on messaging that can move polls in their favor.

President Barack Obama and Republican nominee Mitt Romney will freeze their barrage of television ads for the day though, as ceremonies across the United States remember the 2,977 people killed by the 2001 attacks.

It is recognition of the awkwardness of throwing political punches on a day when politicians typically call for the nation to come together.

“Both campaigns have to be very, very careful to appear respectful and avoid the impression of exploiting a national tragedy for political purposes,” said Bill Galston, a former adviser to President Bill Clinton and fellow at the Brookings Institute.

Romney may be the candidate who most needs to shore up his national security credentials, both as the challenger and because he was criticized even by senior Republicans for omitting to mention the war in Afghanistan during his acceptance speech at the Republican convention last month.

On Tuesday morning, Obama and first lady Michele Obama will join a moment of silence on the South Lawn at the White House and then appear at a memorial service at the Pentagon. Obama will also visit with wounded soldiers and their families at Walter Reed National Military Medical Center.

Romney plans to address the National Guard Association Convention in Reno, Nevada, and campaign staffers in at least six states will collect donations and organize a letter-writing campaign for US troops serving overseas.

Romney's wife, Ann, is expected to participate in events marking the anniversary in Florida. Romney's campaign will halt operations in Virginia, home to the Pentagon where 184 people were killed in the al Qaeda attacks.

“Team Virginia will not engage in political activity on September 11th,” Sara Craig, campaign manager in the battleground state, told supporters on Monday.

Vice President Joe Biden will speak at the Flight 93 memorial in Shanksville, Pennsylvania - another battleground state - where a hijacked United Airlines passenger flight crashed.

While the economy remains the foremost issue among voters, the Obama campaign has been increasingly on the offensive when it comes to foreign affairs.

At the Democratic convention last week, Obama's supporters spoke proudly of bin Laden's killing in 2011, highlighting it as a signature achievement for the president.

When accepting the Democratic presidential nomination, Obama attacked Romney as a foreign policy novice and pointed to the planned end of the war in Afghanistan and the rebuilding of One World Trade Center in downtown Manhattan as signs of American resurgence during his first term.

Romney has praised the US killing of bin Laden, but said Obama's policies toward Iran had made the country less safe during his time in office.