With solemn tributes, the United States is marking the 10th anniversary of the September 11 terrorist attacks that killed nearly 3,000 people.

Events were held at each of the sites attacked a decade ago — in New York City, the Pentagon outside Washington, and Shanksville, Pennsylvania.

U.S. President Barack Obama, leading the nation in remembrances of the tragic anniversary, made a somber tour of all three sites.

In New York, he and predecessor George W. Bush walked through the recently finished memorial at the site of the World Trade Center. Both presidents participated in a ceremony in which family and loved ones read the names of the thousands who were killed when two hijacked jetliners struck the north and south towers of the Trade Center.

Mr. Obama later traveled to Shanksville, Pennsylvania where he and first lady Michelle Obama laid a wreath in honor of the 40 lives lost when Flight 93 crashed into a field outside the rural town. The president and his wife greeted the victims' loved ones and visited the memorial wall inscribed with each passenger and crew member's name.

Sunday afternoon, the president laid a wreath at the Pentagon memorial commemorating the 184 victims of Flight 77's crash into the massive Defense Department headquarters. While hymns played quietly, Mr. Obama and his wife visited with the victims' families gathered at the memorial .

The Obamas will later attend a “A Concert for Hope” in Washington .

Sunday's ceremonies have been marked by moments of silence. In New York, attendees fell quiet at 8:46 a.m. and again at 9:03 a.m. when each of the planes struck the twin towers ten years ago. Attendees later paused to remember when each of the towers fell, and they listened quietly as family and loved ones of the victims read the names of those killed.

After the first moment of silence in New York, President Obama read a Bible passage that speaks of God as refuge and strength. After the second moment of silence, former president Bush read a letter of support from then-president Abraham Lincoln to a mother who had lost all five of her sons in America's Civil War.

The Pentagon observed its moment of silence at 9:37 a.m. local time. At the Pentagon, Vice President Joe Biden spoke of the bravery of first responders and those who joined the military after September 11, 2001, serving in the subsequent wars in Afghanistan and Iraq and other dangerous areas. Defense Secretary Leon Panetta said the events of September 11 have since strengthened and inspired the country.