An attack on Wednesday on a bus carrying US Air Force personnel about to fly home from Germany left two airmen dead and two seriously injured, local authorities and the US military said.
"Two airmen have been killed and two were wounded during a shooting incident today at Frankfurt International Airport," the US Air Forces in Europe (USAFE) said in a statement.
"The names of the deceased are being withheld until 24 hours after notification of next of kin. German authorities have the shooter in custody. The incident is currently under investigation."
Authorities in Germany said that the alleged gunman was a 21 year old originally from Muslim majority Kosovo in southeastern Europe who was living in Frankfurt.
Police said it was so far unclear what his motives were.
One of the victims was killed inside the bus and one outside the vehicle, Boris Rhein, interior minister of the western German state of Hesse, told reporters at the scene.
He added that the alleged killer, who police said used a handgun and who reports said also had a knife, was seized by German police inside the Terminal Two building at the airport, one of Europe's busiest.
"Everything happened on board the bus," police spokesman Juergen Linker told AFP. "The suspected gunman has been arrested. There are two dead and two seriously injured."
Another police spokesman, Andre Sturmeit, told AFP that the injured had been taken to a civilian hospital, but he was unable to give any more information on their condition. He was also unable to say how many people were on the bus.
"The German government will do all it can to investigate what happened," Chancellor Angela Merkel told reporters in Berlin as she expressed her condolences to the friends and families of the deceased.
The scene was sealed off after the attack, an AFP reporter at the scene said, with the bus shielded from view by a screen.
The German police and fire brigade were present, but no one from the US military police was visible.
The US military has a number of major bases near Frankfurt, including the Ramstein airbase, which are used as hubs for operations in Afghanistan and Iraq. Germany, which opposed the 2003 US led invasion of Iraq but has more than 4,000 troops in Afghanistan, has beefed up security and surveillance in response to the threat of attacks in recent years.
The September 11, 2001, attacks on the United States were planned in part in the German port city of Hamburg by an Al Qaeda cell led by Mohammed Atta, the hijacker of the first plane to strike New York's World Trade Center.
In March 2010 a German court jailed four Islamic militants who dreamed of "mounting a second September 11", for a thwarted plot to attack US soldiers and civilians in Germany.
The men from the Sauerland cell, named after the region where three of them were captured in September 2007, admitted to belonging to a "terrorist organisation", plotting murder and conspiring for an explosives attack.