Thousands of people have gathered in New York, Washington and Pennsylvania to mark the 11th anniversary of the September 11 al-Qaida terrorist attacks that killed nearly 3,000 people in the United States.

At the White House, President Barack Obama, his wife Michelle, and other dignitaries bowed their heads in a moment of silence at 8:46 a.m. EDT, the time on September 11, 2001 when the first of two jetliners struck the World Trade Center in New York.

The president later led a ceremony at the memorial at the Pentagon, the headquarters of the U.S. military that was struck by a third plane in the attacks.

Obama stood in front of a huge American flag draped across the building. He told the families of victims that no matter how many years passed, their loved ones would never be forgotten.

In New York, the families of those killed took part in a ritual reading of the victims’ names at Ground Zero, the site of the World Trade Center towers that crumbled after being struck by the planes.

Some people held up photos of their loves ones and cried during the observance.

Vice President Joe Biden is speaking at a ceremony in Shanksville, Pennsylvania, where a fourth hijacked jetliner crashed after passengers tried to take control of the plane, preventing it from possibly reaching another target in Washington.

Republican presidential challenger Mitt Romney is marking the anniversary in a speech to the National Guard Association convention in Reno, Nevada.

U.S. Defense Secretary Leon Panetta on Monday visited the memorial in Shanksville dedicated to the passengers aboard United Flight 93. He said the country must never forget the military personnel deployed to Afghanistan in 2001 to battle Taliban forces who gave shelter to al-Qaida and its leader, Osama bin Laden.

“My concern is that too often, we do not express our concern and our attention for those that are fighting and dying for this country. We’re continuing to lose good men and women in battle in Afghanistan,” said Panetta. “They are putting their lives on the line every day. And, every day, they are fighting to make sure that this country is protected. We cannot forget that sacrifice.”

In a related development, New York Mayor Michael Bloomberg announced a deal had been reached to resume construction of the September 11 museum on the grounds of the World Trade Center. The project had been halted by a dispute between the foundation controlling the museum and the Port Authority of New York and New Jersey, the agency jointly headed by the governors of New York and New Jersey that owns the World Trade Center site.

Also on Monday, federal health officials announced they will add about 50 types of cancer to the list of diseases that will be covered under a special fund established to provide health care to people who became sick while working among the smoldering rubble of the World Trade Center.