Via ABC News:
An ethnic Albanian from Kosovo was taken into custody and the FBI was heading an investigation because U.S. citizens were killed and to determine whether the shooting was an act of terrorism.
Sources told ABC News that the gunman boarded a shuttle bus at the Frankfurt airport. The bus was marked United States Air Force and was carrying 13 or 14 people, plus the driver.
When he opened fire, the gunman shouted "Allah Akbar," according to sources. He fired nine times, killling two and critically wounding two others before the gun jammed and he was subdued by other passengers. While being wrestled into submission, the suspect shouted either "Jihad Jihad" or "Allah Akbar," sources said.