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.

President Barack Obama and his predecessor George W. Bush Sunday morning toured the recently finished memorial at the site of the World Trade Center in New York City. Families of the victims later will be allowed in for the first time to view the reflecting pools that lie where the twin towers once stood.

President Obama and first lady Michelle Obama, along with Mr. and Mrs. Bush, greeted current and former regional officials.

At 8:46 a.m. , New York City held a moment of silence observing the time the first plane struck the North Tower. Another moment of silence will be held at 9:03 a.m. to mark when the second plane struck the South Tower. Attendees at the ceremony also will pause to mark when each of the towers fell.

Following the ceremony at the World Trade Center, the Obamas will head to Shanksville, Pennsylvania, for an observance of the crash of Flight 93.

Later, they will travel to the Pentagon, just outside Washington, for a wreath-laying ceremony honoring the victims of the attack on the massive Defense Department building.

Sunday evening, Mr. and Mrs. Obama will attend a “Concert of Hope” at the Kennedy Center for the Performing Arts in Washington.