German federal prosecutors said Thursday they suspected an "Islamist motivation" was behind the fatal shooting of two US airmen on a military bus at Frankfurt airport.

"In view of the circumstances, there is a suspicion that this was an act with Islamist motivation," prosecutors said, as police questioned a 21-year-old suspect of Kosovan origin arrested after Wednesday's attack.

The man, named in media reports as Arid U., also left two US airmen seriously injured as he opened fire in the midafternoon attack at one of Europe's busiest airports. He was arrested in the terminal building.

German media reports have reported growing evidence that the alleged gunman was an extremist.

Federal prosecutors, who in Germany are responsible for "terrorism" cases and who took over the investigation on Thursday, said the suspect would be brought before a magistrate later on Thursday.

A "saddened and outraged" US President Barack Obama said Washington would "spare no effort in learning how this outrageous act took place," while Chancellor Angela Merkel promised Germany would "do all it can."