Eight years after the attacks that led to the war in Afghanistan, President Obama -- the war's new commander in chief -- commemorated his first 9/11 anniversary as president with a moment of silence at the White House and remarks at the Pentagon.
Obama attended a small, private gathering of victims' families at the Pentagon and laid a wreath at the outdoor memorial with Defense Secretary Robert Gates.
"Eight Septembers have come and gone. Nearly 3,000 days have passed; almost one for each of those taken from us," he said on a cool, rainy day in the Washington area. "But no turning of the season can diminish the pain and the loss of that day. No passage of time and no dark skies can ever dull the meaning of this moment."
The president spoke directly to the families of victims gathered for a memorial service at the Pentagon. "[N]o words can ease the ache of your heart. No deeds can fill the empty places in your homes," he said. "But on this day and all that follow, you may find solace in the memory of those you loved and know that you have the unending support of the American people."
Earlier today the president and first lady Michelle Obama marked a moment of silence with staff at the White House at 8:46 a.m., the moment the first plane struck the World Trade Center on Sept. 11, 2001.
Obama signed an order earlier this year declaring Sept. 11 as a national day of service and said that today Americans can "summon once more that ordinary goodness of America to serve our communities, to strengthen our country and to better our world."
"On a day when others sought to sap our confidence, let us renew our common purpose; let us remember how we came together as one nation, as one people, as Americans, united not only in our grief but in our resolve to stand with one another, to stand up for the country we all love," the president said.
Last year, 20,000 people attended the dedication of the Pentagon memorial where 184 benches with the names of each victim mark the spot where American Airlines flight 77 slammed into the building killing 59 on the plane and 125 on the ground.
The anniversary is a poignant one for Obama, who was just an Illinois state legislator in 2001. Little did he know that eight years later Osama Bin Laden would still be on the run, while the mastermind of the attacks, Khalid Sheik Mohammed, would be growing old in a Guantanamo prison cell.
The president Thursday promised to "apprehend all those who perpetrated these heinous crimes, seek justice for those who were killed and defend against all threats to our national security."
Central to that effort is the war in Afghanistan, which Obama has called a "war of necessity" and to which he has pledged more resources.
But with conditions there worse than ever and American casualties in August, the highest since the war began, the president is now faced with the likely prospect of U.S. commanders asking for tens of thousands of more troops.
Whether he will send them is far from certain. The White House remains deeply divided on the issue and support in Congress for a troop increase appears to be slim.
On the eve of today's anniversary, Speaker of the House Nancy Pelosi, D-Calif., had blunt words about the prospect of shipping even more Americans to war, saying, "I don't think there's a great deal of support for sending more troops to Afghanistan, in the country or the Congress."
Sen. Carl Levin, chairman of the Senate Armed Services Committee, has also expressed skepticism about sending more combat troops until the U.S. does more to train Afghan security forces.
At a park near ground zero in New York today, volunteers who supported the recovery effort will join family members and rescue workers to read the 2,752 names of victims and mark four moments of silence -- for when the planes struck each tower and for when each tower collapsed.
In New York City, Vice President Joe Biden laid flowers in a reflecting pool at a memorial site near ground zero and spoke to the victims' families gathered in remembrance.
"There's a special fraternity of those of us who've lost spouses and children," said Biden whose daughter and first wife died in a 1972 automobile accident. "But there is also one thing all Americans know to be true and which we remember most when we come to this site: in our jobs and in our sorrows, we know we belong to one another."
Biden, who was joined by his wife Jill, read a quote from the poet Mary Oliver about loss and recovery.
Meanwhile, construction of memorial grounds and the U.S. National September 11 Museum continues at ground zero, which planners say will be open to the public by Sept. 11, 2012. Officials called for public participation Thursday in building the museum's collection by submitting images, video and personal stories to the institution's Web site www.911history.org.
In rural western Pennsylvania, the National Park Service is preparing to break ground on the Flight 93 National Memorial, replacing a temporary memorial at the site where hijacked flight United 93 crashed into a field. Former Secretary of State Colin Powell is expected to speak to family and friends of the 40 victims of United 93 at a ceremony at the crash site today.
This is the first year Sept. 11 has been declared a national day of service, and many Americans across the country have planned community improvement projects and remembrance ceremonies to mark the day.
"We must build a culture of resiliency and guard against complacency, so we are better prepared for terrorist attacks or disasters of any kind," said Homeland Security Secretary Janet Napolitano.
"By serving our communities and our country today and throughout the year, we commemorate our past while also preparing for our future," she said.
ABC News' Devin Dwyer and Luis Martinez contributed to this report.