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," 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, and work is under way for four new towers, forcing the ceremony to be moved away from the twin towers' footprints for the first time.

Kathleen Mullen, whose niece Kathleen Casey died in the attacks, said the park is 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."

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

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. Watch 9/11 memorial ceremonies from New York City »

Many of those rescuers are now ill with respiratory problems and cancers, and they blame the illnesses on exposure to the fallen World Trade Center towers' toxic dust.

Also for the first time, the name of a victim who survived that dreadful day 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 who was working a block from the World Trade Center, became the 2,974th victim linked to the attacks.

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, New Jersey, 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. See how people are honoring 9/11 victims »

In New York, drums and bagpipes played as an American flag saved from the WTC rubble 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.

Democratic Sen. Hillary Rodham Clinton, seeking her own party's presidential nomination, also planned to attend ceremonies at ground zero.

President Bush is spending the day in Washington. He attended a private prayer service at St. John's Episcopal Church and 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, a memorial ceremony was also planned.

For the first time since 2001, September 11 is on a Tuesday, the same day as the attacks, but unlike the clear blue sky that morning in 2001, New York was rainy and gray for the anniversary.

In addition to first responders, others to read victims' names include city workers who participated in the cleanup, construction workers and medical examiner's officials who recovered remains and volunteers who helped serve meals at a church across from ground zero.

The ceremony was moved off the trade center site this year because of increased construction at ground zero. The victims will be remembered in a nearby park just southeast of the site, while thousands of family members planned to descend briefly below street level to lay flowers at a spot near the fallen twin towers.

After threats by family members to boycott the ceremony and hold their own shadow remembrance, the city relented and agreed to let them briefly into the pit to lay flowers on the dusty bedrock.

Several family members worried that Zuccotti Park, just southeast of ground zero, would be too small to accommodate the thousands of people expected. City officials said there was actually more space available than at the previous location, and that fewer people have attended the ceremony each year.

Some New Jersey communities that lost many people in the attacks said their ceremonies were being scaled back, and a local television station, WABC-TV, initially decided not to air the four-hour-plus ceremony live. Station officials reversed the decision when viewers complained.

The total number of victims killed at the World Trade Center site is 2,750. Forty were killed in Pennsylvania and 184 died at the Pentagon. Those numbers do not include the 19 hijackers.

National intelligence director Mike McConnell said Tuesday that U.S. authorities are worried about "sleeper cells" of would-be terrorists inside the United States and are remaining vigilant against any new attacks.

McConnell also said plots against the United States have been thwarted. "We're safer, but we're not safe," he said in an interview on ABC's "Good Morning America."

McConnell said that Osama bin Laden's al Qaeda terrorist network "intends to have an operation in the United States that will result in mass casualties."

"We have stopped some efforts and we must stop all efforts. We're not sure we can stop 100 percent of them," he said.

(Published by CNN, September 11, 2007)

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