A gunman is in custody after shooting dead two US airmen and seriously wounding two others in a US Army bus at Frankfurt airport, authorities said.
Security round the airport was tightened and an investigation into the "terrible, senseless crime" was under way, said Boris Rhein, the interior minister for the state of Hesse.
"Whether the incident was linked to terrorism I cannot say at this stage," he told journalists.
The gunman was apparently a Kosovo national, he said. Police said he was 21. A spokesman for Frankfurt airport operator Fraport FRAG.DE said the shooting took place in a US Army bus in front of Terminal 2.
Authorities in Kosovo believed they knew the identity of the gunman "but we don't have confirmation yet," the Kosovo interior minister Bajram Rexhepi said in Pristina.
The United States 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.
Major Beverly Mock, a spokeswoman for the US Air Force at Rammstein air base in Germany where the bus was heading, said the identities of the dead airmen had not yet been confirmed.
"The German authorities have the shooter in custody," she said.
Chancellor Angela Merkel, speaking in Berlin, told a joint news conference with the Portuguese prime minister Jose Socrates "We don't know the details but I would like to express how upset I am. We have to do everything we can to find out what happened."