He apparently shouted out Allahu Ackbar before opening fire on the bus.
The suspect's uncle, 70-year-old Rexhep Uka, said Uka was born and raised in Germany and was a devout Muslim who worked at the airport.The airport is routinely used by US servicemembers as it is a major hub near major US bases in Germany, including those that provide access to the theaters of operation in Afghanistan and Iraq.
He said Uka's grandfather was a religious leader at a mosque in the village of Zhabar, near Mitrovica, Kosovo.
Murat Uka, who identified himself as the alleged shooter's father, says the only thing he knew about his son was that he hadn't turned up at work on Wednesday.
The Frankfurt airport refused to comment on whether the suspect worked there.
The airport, continental Europe’s second biggest after Paris, is routinely used by American soldiers based in Germany for arrivals and departures.
ABC News reported that when the gunman opened fire, he shouted 'Allah Akbar' which is Arabic for 'God is Great'.
The death toll could have been much worse but for the fact that Uka's gun jammed after firing nine times, allowing police to take Uka down.