"Today, we remember not only those who died that September day, we pay solemn tribute to more than 6,700 patriots who have given their full measure since, military and civilians," Obama said in his remarks at a 9/11 observance ceremony at the Pentagon Memorial on the 12th anniversary of the attacks.
"We see their legacy in the friendships they forged, the attacks they prevented, the innocent lives they saved, and in their comrades in Afghanistan who are completing the mission and who, by the end of next year, will have helped to end this war," Obama said.
"Together we pause and we pray and we give humble thanks, as families and as a nation, for the strength and the grace that from the depths of our despair has brought us up again, has revived us again, has given us the strength to keep on," he said.
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As many as 184 people died when a plane hijacked by terrorists crashed into the Pentagon - the headquarters of the US Department of Defense.
"Twelve years ago, at this hour, in this place, a horrific act of terror claimed 184 innocent lives. Today, we come together to honour the fallen, surrounded by those who loved them. We remember each of those taken from us. We remember them as individuals with their own story," Hagel said.
Earlier, at the South Lawn of the White House, Obama, First Lady Michelle Obama, and Vice President Joe Biden observed a moment of silence at precisely 8:45 am (time of the first attack) to remember the victims of the world's deadliest terrorist attacks.