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An armed man waves his rifle as buildings and cars are engulfed in flames inside the U.S. consulate compound in Benghazi late on Sept. 11, 2012. While Stevens, his aide Sean Smith, and former Navy SEALs Glen A. Doherty and Tyrone S. Woods tried to escape, attackers opened fire — killing all four.

Stevens was the first U.S. ambassador killed in the line of duty since Adolph Dubs died in Afghanistan in 1979. Stevens had assisted rebels during their successful uprising against Libyan dictator Moammar Gadhafi during the Arab Spring.

At the arrival ceremony for the remains of the four Americans, Obama said the United States will never pull back on its principles or "retreat from the world."

"Their sacrifice will never be forgotten," Obama said. "They knew the danger, and they accepted it. ... They didn't simply embrace the American ideal. They lived it."

Photo: STR | AFP | Getty Images