Prosecutors in Germany say they suspect Islamic militancy as the motive for a gun attack on US airmen at Frankfurt Airport, in which two died.
The suspected gunman, who is in police custody, is of Kosovo Albanian origin.
He allegedly fired on an air force bus, killing the driver and another man, and injuring two others.
The names of the dead men, who were based at RAF Lakenheath in eastern England, were not released, pending notification to their families.
They had just flown in from Britain and were about to travel to the nearby US air base at Ramstein, a hub for operations in Iraq and Afghanistan.
One airman with a gunshot wound to the head remained in critical condition at the Frankfurt University clinic on Thursday while the other injured man was not in a life-threatening condition, police said.
US President Barack Obama has condemned the shooting as an "outrageous act", and pledged to "ensure that all the perpetrators are brought to justice".
'Engaged in Holy War'
The suspect has been named as Arid Uka, 21, who is said to have been born in Germany to parents who moved there from Kosovo about 40 years ago.
He grew up in Germany and was apparently working at the airport.
Relatives in Kosovo were quoted by the Associated Press news agency as saying that he was a "devout Muslim".
According to German newspapers, he portrayed himself on the internet as an Islamist.
He saw "himself engaged in Holy War with infidels", according to Die Welt, while the Frankfurter Allgemeine Zeitung reports that there were rumours he had planned to embark on a "killing spree".
German police said the shooting appeared to have occurred inside the bus, and the attacker seemed to be acting alone.
The attack took place at roughly 1520 local time (1420 GMT) on Wednesday, just outside Terminal 2 in a bus lane open to the public.
Police believe the gunman used a pistol or revolver.
Four Islamic militants were convicted in March last year in Germany for plotting to bomb targets including Ramstein air base.