Former U.S. presidents George W. Bush and Bill Clinton are in Shanksville, Pennsylvania, for a ceremony dedicating a memorial to the victims of United Flight 93, which crashed into a field during the September 11th, 2001 attacks.

The memorial is made of white stone and commemorates the efforts by passengers and crew of Flight 93 to overcome their hijackers and force the plane to the ground.

Mr. Bush said the passengers' decision to fight back inspired a “spirit of service” in other Americans in the aftermath of the attacks. He also called it the “first counteroffensive in the war on terror.”

Mr. Clinton thanked Mr. Bush and the current administration for protecting the country against further attacks. He, too, praised the 40 passengers and crew members for their actions, saying simply, “They did the right thing.”

Vice President Joe Biden was also present at the ceremony.

In Washington U.S. President Barack Obama is marking the 10th anniversary of the attacks with tributes to the first responders, the military and the thousands of people who lost their lives.

During his weekly address Saturday, Mr. Obama spoke of the acts of heroism a decade ago in New York at the World Trade Center, the Pentagon outside Washington and on United Flight 93.

Later in the day, Mr. Obama and his wife Michelle visited Arlington National Cemetery, where hundreds of thousands of members of the U.S. military are buried. A White House official said the president and his wife believe it is important for all Americans to honor those who have served in the military since September 11, 2001.

The Obamas were visiting a section of the cemetery containing the graves of soldiers killed in the past decade's wars in Iraq and Afghanistan.