(CBS/AP) Relatives of World Trade Center victims bowed their heads in silence at a small park Tuesday to mark the moment exactly six years earlier when the first hijacked plane struck the towers.

The moments of the attacks were remembered by bells and silence, reports CBS News correspondent Jim Taylor (audio).

The dreary, gray skies created a grim backdrop, and a sharp contrast to the clear blue of that morning in 2001.

"That day we felt isolated, but not for long and not from each other," Mayor Michael Bloomberg said as the ceremony began. "Six years have passed, and our place is still by your side."

Construction equipment now fills the vast city block where the World Trade Center once stood. The work is under way for four new towers forced the ceremony to be moved away from the twin towers' footprints for the first time.

"That vision of the clouds over Washington and New York are still haunting our psyche, and we are living in the era of homeland security in the United States right now," presidential historian Douglas Brinkley told CBS News.

As people clutched framed photos of their lost loved ones, Kathleen Mullen, whose niece Kathleen Casey died in the attacks, said the park across the street was close enough.

"Just so long as we continue to do something special every year, so you don't wake up and say, 'Oh, it's 9/11,'" she said.

Presidential politics and the health of ground zero workers loomed over the sixth anniversary of the terrorist attacks this year, perhaps more than any other Sept. 11.

"We need to put our arms around the people that have actually both suffered injury and the families that were involved. I don't think this is a short-run affair," New Jersey Gov. Jon S. Corzine told WCBS-TV.

The firefighters and first responders who helped rescue thousands that day in 2001 and later recovered the dead were to read the victims' names for the first time. Many of those rescuers are now ill with respiratory problems and cancers themselves, and they blame the illnesses on exposure to the fallen towers' toxic dust.

Also for the first time, the name of a victim who survived that towers' collapse but died five months later of lung disease blamed on the dust she inhaled was added to the official roll.

Felicia Dunn-Jones, an attorney, was working a block from the World Trade Center. She became the 2,974th victim linked to the four attack sites where hijacked airliners hit the two towers, the Pentagon and a field near Shanksville, Pa., where federal investigators say the passengers of United Airlines Flight 93 fought the hijackers on the rallying cry "Let's roll!"

"As American citizens we're all looking at our heroes," said Kay Roy, whose sister Colleen Fraser, died in the crash over Pennsylvania. "These are our heroes and I'm glad that one of my family members happens to be one of these heroes."

In the Boston Public Garden, Mayor Tom Menino placed a wreath of red, white and blue flowers at a granite memorial to the 9/11 victims. In attendance were about two dozen relatives of Bay State residents killed in the attacks. Two of the airliners used in the attacks took off from Logan Airport.

Bells from the nearby Arlington Street Church rang out "Amazing Grace" and "America the Beautiful" during the otherwise silent ceremony.

At the Pentagon, Gen. Peter Pace, chairman of the Joint Chiefs of Staff, spoke at the wall where one plane crashed and told the victims' families that their loved ones will be remembered, reports CBS News correspondent Cami McCormick (audio).

"I do not know the proper words to tell you what's in my heart, what is in our hearts, what your fellow citizens are thinking today. We certainly hope that somehow these observances will help lessen your pain," he said.

At the main U.S. base at Afghanistan, service members bowed their heads in memory of the victims.

At the state capitol in Frankfort, Kentucky Gov. Ernie Fletcher led a solemn ceremony to remember the victims of the terror attacks.

In New York, drums and bagpipes played as an American flag saved from the collapse was carried toward a stage.

At night, two massive beams of light will project into the sky above ground zero, reports CBS News Early Show national correspondent Jeff Glor.

Firefighters were to share the stage with former Mayor Rudy Giuliani, who many victims' families and firefighters said should not speak because he is running for president. Giuliani has made his performance in the months after the 2001 terrorist attacks the cornerstone of his campaign, but he has said his appearance wasn't intended to be political.

"I was there when it happened and I've been there every year since then. If I didn't, it would be extremely unusual. As a personal matter, I wouldn't be able to live with myself," Giuliani said late last week.

Sen. Hillary Rodham Clinton, seeking the Democratic Party presidential nomination, also planned to attend the ceremonies at ground zero.

President Bush, with the first lady at his side, held a moment of silence on the South Lawn of the White House, after attending a private church service in Washington, reports CBS News correspondent Peter Maer (audio). At the main U.S. base at Afghanistan, a memorial ceremony was also planned.

As in past years, moments of silence were planned to mark each crash and the collapse of each tower in New York.

"Six years after the event, it's still dominating our lives, because it changed the whole tone and tenor of behavior," said Brinkley. "Anybody that's going to an airport or trying to get on a subway or just simply having to walk into a business building and flashes their ID are in a way victims of what happened during 9/11."

Mail still arrives addressed to the World Trade Center, about 200 pieces a day, reports CBSNews.com's Lloyd de Vries (audio). There's less each year, and chances are it's coming from companies that still haven't updating their mailing lists. Most of it is returned to the mailers, although some former tenants pay a fee to have the incoming mail held and then pick it up.

Even though the World Trade Center ceremony gathering was in the park, thousands of family members were still allowed to descend briefly below street level to lay flowers at a spot near the twin towers' footprints. Family members upset that they might not be allowed in at all pressured the city to at least allow the short visits to the dusty bedrock.

In all, 2,974 victims were killed by the Sept. 11 attacks: 2,750 at the World Trade Center, 40 in Pennsylvania and 184 at the Pentagon. Those numbers do not include the 19 hijackers.

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