Relatives of Sept. 11 victims bowed their heads in silence Tuesday to mark the moments exactly six years earlier when hijacked planes crashed into the World Trade Center, the Pentagon and a Pennsylvania field. The dreary 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,” New York Mayor Michael Bloomberg said as the first 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 under way for four new towers forced the ceremony’s move away from the twin towers’ footprints and into a nearby park for the first time.

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

“We’re still very much affected by it on a daily basis,” said Tania Garcia, whose sister Marlyn was killed. “It’s an open wound, and every year that passes by just get worse and worse and worse.”

This anniversary, presidential politics and the health of ground zero workers loomed, perhaps more than any other.

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.

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 crashes of the hijacked airliners in New York, 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!”

A memorial honoring Flight 93’s 40 passengers and crew began at 9:45 a.m., shortly before the time the airliner nosedived into the empty field.

“As American citizens we’re all looking at our heroes,” said Kay Roy, whose sister Colleen Fraser, of Elizabeth, N.J., died when the plane went down. Homeland Security Secretary Michael Chertoff planned to speak to the mourners.

In Boston, where two of the hijacked airplanes took off that morning, church bells rang to the tunes of Amazing Grace and America the Beautiful.

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

At the Pentagon, Gen. Peter Pace, the chairman of the Joint Chiefs of Staff, spoke at the wall where the plane crashed and told the victims’ families that their loved ones will be remembered.

“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.

Pace also spoke of the military, calling the anniversary “a day of recommitment.” At the main U.S. base at Afghanistan, service members bowed their heads in memory of the victims.

National intelligence director Mike McConnell said U.S. authorities remain vigilant and concerned about “sleeper cells” of would-be terrorists inside the United States.

“We’re safer but we’re not safe,” McConnell said on ABC’s “Good Morning America.”

Even though the World Trade Center ceremony gathering was moved out of ground zero, thousands of family members descended briefly into the site to lay flowers near the twin towers’ footprints.

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

On the Net:

http://www.familiesofseptember11.org