Two US Air Force servicemen have died after a gunman opened fire on a bus carrying American military personnel at Frankfurt Airport, police say.
Two servicemen were also wounded. One of the dead is said to have been the driver. All four were based at RAF Lakenheath in eastern England.
A 21-year-old man whose family is from Kosovo was arrested at Terminal 2 close to where the attack took place.
US President Barack Obama condemned it as an "outrageous act".
He said the US would spare no effort in investigating the shootings to "ensure that all the perpetrators are brought to justice".
"I think the American people are united in expressing our gratitude for the service of those who were lost," Mr Obama said.
The men had just flown in from Britain and were about to travel to the US military base at Ramstein, a hub for operations in Iraq and Afghanistan.
"Two airmen have been killed and two were wounded during a shooting incident today at Frankfurt International Airport," the US Air Forces in Europe 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," the statement added.
The bus driver and a passenger were killed, while one airman suffered minor injuries and the second was in a life-threatening condition, according to Frankfurt police spokesman Manfred Fuellhardt.
Mr Fuellhardt said the attacker and US military personnel apparently had an altercation in front of the bus just before the man started shooting. The attacker briefly entered the bus, he added.
The BBC's Stephen Evans in Berlin says that after the shooting, which took place in the middle of the afternoon, the suspected gunman fled into the terminal building, where he was arrested by German police.
Security has been high in German airports in recent months, following what the authorities said was information that a terror attack was planned, but the official threat level was lowered a month ago, our correspondent adds.
Kosovo Interior Minister Bajram Rexhepi said in an interview that German police had identified the suspect as a Kosovan citizen from the northern town of Mitrovica.
Relatives in Mitrovica named him as Arid Uka, whose family had moved to Germany 40 years ago.
"This is a devastating and a tragic event. We are trying to find out was this something that was organised or what was the nature of the attack," Mr Rexhepi said, reports AP news agency.
German Chancellor Angela Merkel condemned the "terrible incident".
"I would like to assure you that the German government will do our utmost to investigate what happened," she told journalists in Berlin.
Four Islamic militants were convicted in March last year in Germany for plotting to bomb targets including Ramstein Air Base.
Last month the German parliament extended by one year the military mission in Afghanistan. Germany has 4,860 troops there, despite domestic polls suggesting the mission's unpopularity.