Two US airmen have been shot dead by a lone gunman outside Frankfurt Airport.
Two others were injured in the attack on a US services bus outside a terminal.
German police have identified the gunman as a Kosovo citizen but the motive for the attack was not immediately clear.
The bus driver and a passenger were killed, and one person suffered serious wounds and another light injuries, an airport spokesman said.
A US Air Force Europe spokeswoman confirmed that all four victims were airmen.
In Berlin, 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.
Kosovo Interior Minister Bajram Rexhepi said the gunman was Arif Uka, a Kosovo citizen from the northern town of Mitrovica.
"This is a devastating and a tragic event," Mr Rexhepi said. "We are trying to find out was this something that was organised or what was the nature of the attack."
A state official who went to the scene said there were no indications of a terrorist attack.