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.