United States President Barack Obama on Tuesday paidtribute to those killed in the September 11, 2001, terrorist attacks,noting at a ceremony at the Pentagon that although 11 years hadpassed the pain remained fresh for the victims' families.

"Eleven times we have marked another September 11 come and gone. Eleven times we have paused in remembrance and reflection, in unityand purpose. This is never an easy day but is especially difficultfor all of you," he said to family members gathered at the memorialwhere a plane slammed into the heart of U.S. military power.

The world had changed because of the nearly 3,000 people killed atthe World Trade Center in New York, at the Pentagon and inPennsylvania, he said, pointing to the killing of al-Qaida leader Osama bin Laden and the war in Afghanistan.

“Most had never heard the name al-Qaida. And yet it's because oftheir sacrifice that we've come together and dealt a crippling blowto the organization that brought evil to our shores," he said.

“Al-Qaida's leadership has been devastated, and Osama bin Laden willnever threaten us again.”