NEW YORK/WASHINGTON, Sept. 11 -- Americans, led by President Barack Obama, on Thursday commemorated 13th anniversary of the Sept. 11, 2001 attacks by observing moments of silence for thousands of innocent people killed that day at the World Trade Center of New York City, the Pentagon and a Pennsylvania field.

As an annual ritual, relatives began reciting names of the nearly 3,000 victims killed at a ceremony held Thursday morning in lower Manhattan, New York City. They paused six moments after a silver bell was rung to mark the timing when each plane hit the twin towers and when each tower fell, as well as the timing of the attacks on the Pentagon and on Flight 93 that crashed into the Pennsylvania field.

With tolling of the bell, a moment of silence was observed.

While addressing the relatives of the 184 people killed in the attack carried on the Department of Defense Headquarters, President Obama said they had offered "the ultimate rebuke to the hatred of those who attacked us that bright, blue morning."

"Thirteen years after small and hateful minds conspired to break us. America stands tall and America stands still," said Obama. "No matter what comes our way, America will always come out stronger." "We carry on, because, as Americans, we do not give in to fear -- ever."

Before that, President Obama and First Lady Michelle, who were joined by Vice President Joseph Joe Biden and White House staff, also gathered under partly cloudy skies on the South Lawn of the White House and observed a moment of silence at 8:46 a.m. of the day, the time when the first hijacked airliner struck the World Trade Center in New York City.

The National September 11 Memorial and Museum in lower Manhattan, New York, where artifacts and graphic photos on the attacks are displayed, has been opened to the public for the first time since May.

"We remember the names, faces, and lives of the men, women, and children who were killed, and look for ways to ensure that each and every one of them is not forgotten, "said Joe Daniels, president and CEO of the National September 11 Memorial and Museum in New York.

A nearly completed One World Trade Center has risen 1,776 feet, marking the year of America's independence, above the ground zero and is scheduled to open in 2015, a sign of a return to normalcy in the area used to be immersed in grief.

There were other events in commemoration of the 13th anniversary Thursday. The Tribute in Light, a symbolic recreation of the Twin Towers, has returned to lower Manhattan. And for the first time, the public were allowed to view the lights from the 9/ 11 memorial plaza from 6 p.m. until midnight. Also, New Yorkers were expected to mark the anniversary at the twin reflecting pools where the towers once stood.

Though much has changed with the passage of time, the daunting task of uprooting terrorism remains the same.

On the eve of the 13th anniversary of the Sept. 11 attacks, President Obama spoke to the Americans about his strategy to fight terrorism. In his speech, he said he will expand his airstrike campaign against the Islamic State as he sees Islamic militant group as a threat to America. The move has sparked controversy among the U.S. public who are weary of being dragging into another gruesome, decade-long war and suspicious of the effectiveness of hardball tactics in fighting against terrorists.

Though the president vowed to never "give in to fear," several surveys indicated the increasing worries among Americans about extremism in the Middle East. Six in 10 Americans now are very concerned about the rise of Islamic State terrorists around the world, the largest share since 2007, according to a new survey from the Pew Research Center.

Another CNN poll released Wednesday also showed more than half of Americans did not felt that the United States is safer from terrorism than it was before Sept.11, 2001.