The United States is marking the 11th anniversary of the September 11 terrorist attacks that killed nearly 3,000 people.
In New York, relatives of those killed at the World Trade Center towers read out the names of the victims in a ceremony where the towers once stood.
A minute of silence was held at 8:46 a.m., the time when the first of four planes hijacked by terrorists struck the North Tower of the 110-story World Trade Center.
In Washington, President Barack Obama and First Lady Michelle Obama held a minute of silence on the South Lawn of the White House.
The president and first lady later laid a white floral wreath at the Pentagon, at the memorial marking the spot where one of four airliners hijacked by terrorists hit that day 11 years ago at 9:37 a.m.
In remarks at the Pentagon, Obama recalled the horror and the grief of that day.
"So as painful as this day is and always will be, it leaves us with a lesson that no single event can ever destroy who we are. No act of terrorism can ever change what we stand for," Obama said.
"Instead, we recommit ourselves to the values that we believe in, holding firmly, without wavering, to the hope that we confess."
Obama said a stronger United States will eventually emerge from the legacy of the 9/11 attacks.
"When the history books are written, the true legacy of 9/11 will not be one of fear or hate or division," he said. "It will be a safer world, a stronger nation, and a people more united than ever before."
Obama later visited the graves of recent war dead from Afghanistan and Iraq at Arlington National Cemetery.
Vice President Joe Biden attended a ceremony in Shanksville, Pennsylvania, where a hijacked plane crashed after passengers attempted to seize the airliner back from the hijackers.
Speaking on the eve of the anniversary, U.S. Secretary of Defense Leon Panetta vowed the United States will remain vigilant to the threat of terrorism and that anyone who attacks the United States will pay for it.
"Today, we gather here to renew our pledge -- our pledge to those who died on 9/11, our pledge to their families and our pledge to all Americans that we will remain forever vigilant against threats to our homeland, and that nobody -- nobody -- attacks the United States of America and gets away with it," Panetta said.
WATCH: Unforgettable images from September 11, 2001
Fallout Continues, 11 Years After
Many Al-Qaeda leaders have been killed in the last 11 years -- including the terrorist network's head, Osama bin Laden, who was killed by Navy SEALs last year in a raid on his hideout in Pakistan.
Al-Qaeda's current leader, Ayman al-Zawahri, confirmed this week that his deputy, Abu Yahya al-Libi, was killed in Pakistan by a U.S. drone strike in June.
But as the number of killed Al-Qaeda figures grows, so does the number of deaths in the United States from the infamous September 11 attacks.
The U.S. National Institute for Occupational Safety and Health announced on September 10 that illnesses related to the September 11, 2001, attacks continue to claim lives.
Some 1,000 deaths during the past 11 years have been linked to illnesses caused by toxic dust from the collapsed World Trade Center towers.
The New York City Fire Department etched nine more names into the memorial wall honoring firefighters who died from illnesses after their work at the site, bringing the total to 64.
Construction workers, firefighters, policemen, other first responders -- as well as people near the area immediately after the attacks -- have fallen ill since the tragedy.
The U.S. federal government on September 10 added 50 types of cancer to the list of illnesses covered by the 9/11 health-treatment program.
Some 70,000 surviving firefighters, police officers, and other first responders are eligible for free monitoring and treatment.
In a related story, a cost dispute that halted work for months on New York's 9/11 museum was resolved on the eve of the September 11 anniversary.
The National September 11th Museum in New York originally was supposed to have been completed by Septmber 11, but the museum's president said it could take more than a year to finish the job.
The museum will include artifacts from the day of the attacks such as firefighters' helmets and pieces of rubble.
The cost for the project is estimated at up to $1.4 billion.
With reporting by Reuters, AP, and AFP