A lone gunman has opened fire on a US military bus outside Frankfurt airport killing two servicemen and wounding two others.
A 21-year-old Kosovan man fled from the scene but was arrested in a terminal building, local police said. All four casualties were American airmen based in the UK, a US air force spokeswoman said.
Major Beverly Mock, who works at the Ramstein airbase where the bus was heading, said she could not confirm the men's identities until next of kin had been informed.
Security around the airport was tightened and an investigation into the "terrible, senseless crime" was under way, said Boris Rhein, interior minister for the state of Hesse. "Whether the incident was linked to terrorism I cannot say at this stage," he told journalists.
Police said it appeared an argument had broken out on the bus before the suspect opened fire. The body of one soldier was found outside the bus, which had a US government licence plate marked AF, for air force. One of the dead was said to be the driver, and the other a passenger.
The attack occurred around 3.20pm local time as the bus stopped outside the airport's terminal 2, according to Frankfurt police spokesman Manfred Fuellhardt.
One person suffered serious wounds and another less serious injuries, Fuellhardt said. The two injured had been shot in the head and chest, he said.
"It all appears to have taken place on the bus," said Jürgen Linker, also from Frankfurt police. It was too early to discuss possible motives, he added.
Barack Obama said he was "saddened and outraged" by the killings and that the US would spare no effort in finding out they happened. Shortly after the shootings Patrick Meehan, a member of the congressional homeland security committee, said it looked like a terrorist attack and that he was being briefed by his staff. The Pentagon said it had no details about the reason for the attack .
At the airport, cab driver Salimi Seraidon was at a stand about 200 metres away when the attack took place, and said it was over quickly as police rushed to the scene. "We just heard the shots," he said.
Though German police would not name the suspect Kosovo's interior minister, Bajram Rexhepi, said he was Arif Uka, from the town of Mitrovica. "This is a devastating and a tragic event," Rexhepi said. "We are trying to find out was this something that was organised or what was the nature of the attack."
The US has had troops in Kosovo since 1999, when a Nato bombing campaign pushed out Serbian forces. The US troops there now are helping to oversee a fragile peace that has held since Kosovo declared independence from Serbia in 2008.
American troops have been based in Germany since the end of the second world war . Though the US has reduced its forces in Germany over the past decade, it still has about 50,000 troops in the country.
It operates several major facilities in the Frankfurt region, including the Ramstein airbase often used as a logistical hub for operations in Afghanistan and Iraq.
In Berlin, the German chancellor, Angela Merkel, said her sympathies were with the victims and their families, and pledged that Germany would do everything in its power to investigate the crime. "It is a terrible event," she said.
The worst previous attack on US soldiers in Germany was in 1986, when a bomb was planted in a Berlin disco. Two soldiers died along with a Turkish woman, and 230 people were injured. The US blamed Libya and bombed Tripoli and Benghazi. During the 1970s and 80s, the leftwing German terror group Red Army Faction repeatedly targeted US soldiers in Germany.