With solemn tributes, the United States is marking the 10th anniversary of the September 11 terrorist attacks that killed nearly 3,000 people and launched the country into a decade of war.

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

In New York, a moment of silence marked when the first plane struck at 8:46 a.m., and silence fell again at 9:03 a.m. in observance of when the second hijacked jetliner struck the South Tower. Attendees also 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.

At the Pentagon, a silent tribute was held at 9:37 a.m. local time, to remember the 184 victims of the crash of Flight 77 into the massive Defense Department headquarters.

Early Sunday, President Barack Obama and his predecessor George W. Bush toured the recently finished memorial at the site of the World Trade Center in New York.

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 in the rural town. The president and his wife greeted the victims' loved ones and visited the memorial wall that is inscribed with each passenger and crew member's name.

The Obamas later will attend a wreath-laying ceremony at the Pentagon followed by “A Concert for Hope” in Washington .

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.

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.