Two U.S. Air Force servicemen were killed and two others seriously injured Wednesday after a gunman opened fire on their bus outside the Frankfurt, Germany airport.

The 21-year-old gunman, Arid (also spelled Arif) Uka, repeatedly shouted "Allahu Akbar" (Allah is greater) as he fired nine rounds at the men before his gun jammed and other servicemen were able to subdue him, officials said. 

German police identified the shooter as an Albanian from Kosovo whose parents moved to Germany many years ago. He is a devout Muslim who worked at the Frankfurt airport, ABC News reported.

President Barack Obama said he's "saddened and outraged" by the shooting, and the United States would do all it could to find out how such an "outrageous" act took place.

An Air Force spokesman at Lakenheath airfield confirmed all four of the U.S. servicemen were from the military base in eastern England.

The U.S. military has several major bases in the Frankfurt region that are used for operations in Afghanistan or Iraq.

Click play for more on the shooting with CBN News Terrorism Analyst Erick Stakelbeck.