People marked Tuesday the moments exactly six years earlier when hijacked planes crashed into the World Trade Center.
Under a grim backdrop of dreary skies near New York's ground zero, four moments of silence were observed to remember when the two planes struck the World Trade Center towers, and when each tower fell.
"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."
Presidential politics, the health of ground zero workers, and the continuing security threat loomed over the ceremony. Hours before, a video from al-Qaida leader Osama bin Laden was released, calling for sympathizers to join a "caravan" of martyrs. It served as a stark reminder that the United States has failed to catch the man believed to be behind the attacks that killed almost 3,000 people.
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.
A memorial honoring Flight 93's 40 passengers and crew began at 9:45 a.m., shortly before the time the airliner nose-dived into the empty Pennsylvania field.
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 said should not speak at the service to keep from politicizing it given his Republican presidential bid.
Giuliani has made his performance after the 2001 terrorist attacks the cornerstone of his campaign, but he 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 did not 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.
Sen. Hillary Rodham Clinton, seeking the Democratic Party presidential nomination, also attended the ceremonies. Republican Mitt Romney, another presidential contender, issued a statement describing the attacks as the day "radical Islamists brought terror to our shores."
Democratic candidate Barack Obama called for the country to "recapture the sense of common purpose," while saying the "threat to America has only grown." Fellow Democratic presidential hopeful Bill Richardson called on President George W. Bush and Congress to strengthen intelligence gathering and pursue a foreign policy that will eradicate terrorism.
In Washington, Bush paused for a moment of silence outside the White House, while at the Pentagon, Gen. Peter Pace spoke at the wall where the hijacked plane broke through. At the main U.S. base at Afghanistan, service members bowed their heads in memory of the victims.
In a speech to family members of some of the Pentagon victims, Defense Secretary Robert Gates vowed that: "The enemies of America ... will never again rest easy, for we will hunt them down relentlessly and without reservation."
"Here at the Department of Defense, we pay an ongoing tribute with our firm commitment to defend the United States against any and all enemies, wherever they may exist," Gates said.
But, the release of the bin Laden video Tuesday underscored the U.S.'s failure to find bin Laden despite Bush's vow in the wake of the attacks to take him "dead or alive".
The video featured an audiotape introduction by the al-Qaida leader and showed hijacker Waleed al-Shehri addressing the camera. "We shall come at you from your front and back, your right and left," al-Shehri, one of the hijackers on American Airlines Flight 11 which hit the World Trade Center, warns Americans.
"It is true that this young man was little in years, but the faith in his heart was big," bin Laden said, paying his own tribute to the hijacker.
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."
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.
Even so, about 3,500 family members descended briefly into the site to lay flowers near the twin towers' footprints.
Among the first family members down the ramp was Marjorie Miller, whose late husband Joel died in the trade center attacks. She said the rain was almost welcome after five consecutive years of Sept. 11 sunshine.
"A lot of tears coming down from up there," she said, gesturing toward the sky, "and a lot of tears down here."
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.