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(CNN) - After a bell tolled exactly at 8:46 a.m. on Wednesday, Ground Zero in New York City fell silent.
A poignant remembrance as hundreds of people paused to mark the moment when the first jet crashed into the north tower of the world trade center.
The tribute overwhelmed some to tears and moved others to hugs of comfort.
People also held photos of loved ones and friends lost in the terror attack 12-years ago.
Family members of victims of the 1993 bombing of the World Trade Center and the 2001 attack then began a solemn reading of the names of those killed at the site.
President Obama, Vice President Biden and their wives observed a moment of silence at the White House.
President Obama then joined Secretary of Defense Chuck Hagel in a private ceremony at the Pentagon.
"They left this Earth. They slipped from our grasp. But it was written, 'What the heart has once owned and had, it shall never lose,'" Obama said.
Overnight, two spotlights on the New York City skyline honor the people who died in the terror attacks 12 years ago.
"I will always remember walking through lower Manhattan on the day after those attacks," Former Secretary of State Hillary Clinton said. "The air was acrid. The thick smoke made it hard to breathe."
A lot of work has happened in the years since the attacks. Earthcam released this time-lapse video showing the construction at One World Trade Center between October 2004 and September of this year. The tower's completion is expected early next year.
In Pennsylvania, officials broke ground on a new visitor's center near the memorial for victims of United Airlines Flight 93, which hijackers crashed near Shanksville, apparently as passengers and crew members fought back.
Services Wednesday at both sites and at the Pentagon will pay tribute to the 2,977 people who died in those attacks.
Meanwhile, President Barack Obama deals with what he calls another threat to national security - Syria's use of chemical weapons, a proposal to respond with U.S. military strikes.
"Many of you have asked, 'Won't this put us on a slippery slope to another war?' One man wrote to me, 'We are still recovering from our war in Iraq.' A veteran put it more bluntly: 'This nation is sick and tired of war,'" Obama said.
He says Assad's regime cannot seriously threaten the U.S. military. "Any other retaliation they might seek is in line with threats that we face every day," he said.