Frankfurt Airport shooting: Two US servicemen dead
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 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.
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