Those who cheered the attacks are also marking the anniversary. A- one which has not been previously released, featuring both bin Laden and one of the Sept. 11 hijackers - has apparently turned up. A still photo from the video shows hijacker Walid al Shehri, wearing a white turban, superimposed over an image of the World Trade Center in flames.
Our own leaders in the war on terror are marking the day with reports on readiness and action plans, as well as anniversary ceremonies.
On Capitol Hill, 9/11 Commission co-chairs Tom Kean and Lee Hamilton will be holding a news conference on the government's progress in implementing their recommendations. Also Tuesday, a House committee will be hearing from FEMA administrator David Paulison on plans for responding to emergencies.
In Washington, President Bush and Laura Bush will begin the day with a prayer service and will mark the time of the attacks with a moment of silence morning on the South Lawn of the White House.
At the Pentagon, Defense Secretary Robert Gates will host a memorial observance for relatives of those who died in the attack there.
Numerous other ceremonies are being held around the nation and around the world, including at the Pennsylvania field where Flight 93 crashed.
Pennsylvania Gov. Ed Rendell will be among those on hand in Shanksville, where anniversary ceremonies include the unveiling of a U.S. Postal Service postmark commemorating Flight 93.
In New York Tuesday, the anniversary ceremonies will be a few feet away from Ground Zero - where construction on new buildings has made the site too hazardous for large gatherings.
This year, for the first time, firefighters who helped rescue New Yorkers on Sept. 11, 2001 - and later, recovered the dead - will be reading victims' names at an anniversary ceremony.
Many of the first responders have become sick, or died themselves, of respiratory problems and cancers they blame on exposure to World Trade Center dust. For the first time this year, a victim who did not die at the trade center will be recognized - the city added the name of an attorney who died of lung disease five months later to its official victims' list this year.
And firefighters will share the stage with former mayor Rudy Giuliani, who many victims' families and firefighters said should not speak here this year because he is running for president as a Republican.
"He's really kind of raining on their coattails," said Rosaleen Tallon, whose firefighter brother Sean Tallon was killed on Sept. 11.
Once again this year, the city will hold four moments of silence to mark the times that two hijacked jetliners hit the twin towers, and for the two times that each tower collapsed. For the first time since 2001, Sept. 11 is on a Tuesday.
But 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 allow them to descend briefly into the pit to lay flowers on the dusty bedrock.
A fire union spokesman said no organized demonstration of messages are planned against Giuliani, or about the blaze last month that killed two firefighters at a skyscraper that had never been torn down since it was heavily damaged on Sept. 11.
But the toxic tower will be in view of people attending the ceremony, providing another grim reminder of the Sept. 11 legacy six years later.
Giuliani, who has made his performance in the months after the 2001 terrorist attacks the cornerstone of his campaign, said last week that 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 Friday.
Several family members worried that Zuccotti Park, just southeast of ground zero, would be too small to accommodate the thousands of people. City officials said there was actually more space available than at the previous location, and said that fewer people were attending the ceremonies each year.
With this Sept. 11 more than others, more people have been questioning whether anniversary fatigue is setting in.
Some New Jersey communities that lost many people in the attacks said their ceremonies have been scaled back, and a local television station, WABC-TV, initially decided not to air the four-hour-plus ceremony live. The station changed its mind once the public complained.