Family members of Sept. 11 victims marked the 12th anniversary of the attacks Wednesday at ground zero in New York and elsewhere with the reading of names and moments of silence.
At a morning ceremony on the two-year-old memorial plaza in New York, relatives recited the names of the nearly 3,000 people who died when hijacked jets crashed into the World Trade Center, the Pentagon and a site near Shanksville, Pa., as well as names of victims of the 1993 Trade Center bombing.
"No matter how many years pass, this time comes around each year -- and it's always the same," said Karen Hinson of Seaford, N.Y., who lost her 34-year-old brother, Michael Wittenstein, a Cantor Fitzgerald employee.
"My brother was never found, so this is where he is for us," she said as she arrived for the ceremony with her family early Wednesday.
In Washington, President Barack Obama joined first lady Michelle Obama, Vice President Joe Biden, Jill Biden and members of the White House staff in a moment of silence on the South Lawn.
Afterward, in a memorial service at the Pentagon, Obama called on Americans to pray for those whose lives had been lost.
"Let us have the strength to face the threats that endure, different though they may be from 12 years ago, so that as long as there are those who would strike our citizens, we will stand vigilant and defend our nation," he said.
The morning after a speech in which he urged Americans to support his proposal to use military force against Syria, in retribution for President Bashar al-Assad's alleged poison-gas attack on his own people, Obama also reflected on the limits of force.
"Let us have the wisdom to know that, while force is at times necessary, force alone cannot build the world we seek," he said.
Back at the New York memorial, Brittney Cofresi, the niece of a 9/11 victim, urged the president to consider the cost of past conflicts when evaluating the crisis in Syria.
"President Obama, please do not bring us to another war," she said.
At 8:46 a.m., the moment that the first plane crashed into the North Tower of the World Trade Center, many Americans, including the president, first fell silent. It was the first of six moments of silence. At 9:03 a.m., the second plane struck the South Tower. At 9:37 a.m., Flight 77 struck the Pentagon. At 9:59 a.m., Americans watched in horror as the seemingly unshakable South Tower fell. It was followed by the North Tower 29 minutes later, at 10:28 a.m. At 10:03 a.m., Flight 93 crashed into a field in Pennsylvania; the passengers and crew overtook the hijackers, sacrificing their own lives to prevent an attack on the Capitol in Washington, D.C.
Name reading, wreath laying and other tributes were also taking place at the Pentagon and at the Flight 93 National Memorial in Shanksville while the commemoration unfolded at ground zero, where New York Mayor Michael Bloomberg, who has helped orchestrate the observances from their start, watched for his last time in office.
Memorial organizers expect to take primary responsibility for the ceremony next year and say they plan to continue concentrating the event on victims' loved ones, even as the forthcoming museum creates a new, broader framework for remembering 9/11.
"As things evolve in the future, the focus on the remembrance is going to stay sacrosanct," memorial president Joe Daniels said.
Around the world, thousands of volunteers have pledged to do good deeds, honoring an anniversary that was designated a National Day of Service and Remembrance in 2009. Obama is taking part in a service project on Wednesday afternoon.
Al Jazeera and wire services