NEW YORK &#8212 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.

On this sixth 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 also spoke to the mourners, telling them: “You have my promise that we will continue to work every single day to protect the people of this country, all in the name of those who perished heroically on Flight 93.”

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

Firefighters shared the platform with former Mayor Rudy Giuliani, who many victims’ families and firefighters had said shouldn’t speak at the service to keep from politicizing it.

Giuliani has made his performance after the 2001 terrorist attacks the cornerstone of his presidential campaign, but the Republican has said his desire to be there Tuesday was entirely personal.

“It was a day with no answers, but with an unending line of people who came forward to help one another,” he told those gathered.

After he descended to the trade center site, however, one man yelled “Scum! Scum!” at him, and a woman from the family line said Giuliani didn’t allow enough time for the search for victims’ remains.

“Because of Giuliani, we never had closure,” said Sabrina Rivera. Giuliani left without speaking to reporters.