U.S. President Barack Obama, Defense Secretary Ash Carter (unseen) and Joint Chiefs Chair Gen. Joseph Dunford take part in a ceremony marking the 15th anniversary of the 9/11 attacks at the Pentagon in Washington, U.S., September 11, 2016. REUTERS/Joshua Roberts

U.S. President Barack Obama, Defense Secretary Ash Carter (unseen) and Joint Chiefs Chair Gen. Joseph Dunford take part in a ceremony marking the 15th anniversary of the 9/11 attacks at the Pentagon in Washington, U.S., September 11, 2016. REUTERS/Joshua Roberts

U.S. President Barack Obama, Defense Secretary Ash Carter (unseen) and Joint Chiefs Chair Gen. Joseph Dunford take part in a ceremony marking the 15th anniversary of the 9/11 attacks at the Pentagon in Washington, U.S., September 11, 2016. REUTERS/Joshua Roberts

September 11, 2016 2:22 PM (UTC+8)