A lone gunman armed with a handgun attacked a bus carrying U.S. Air Force troops at Frankfurt airport in Germany Wednesday, killing two U.S. airmen and wounding two other American servicemen before being taken into custody, authorities said.

The shooting was carried out by a 21-year-old Albanian Kosovar. The motivation of the attacker, who is in police custody, is currently unclear, but those on the hyper-patriotic reactionary right are already screaming bloody terrorism.

In Washington, President Obama promised to "spare no effort" in investigating the slayings.

Police authorities said they believe the suspect stormed onto the bus, which was waiting at the terminal, and began shooting.

German Chancellor Angela Merkel said. "We don't know the details but I would like to say how upset I am."