German police are investigating whether the fatal shooting of two US airmen at Frankfurt airport was a terrorist attack. Another two American servicemen were wounded when a lone gunman opened fire on a US military bus parked outside the terminal building on Wednesday. Investigators are pursuing what German media described as "substantial evidence" that the 21-year-old Kosovan suspected of carrying out the attack had links with Islamist groups in Germany. There is concern that more attacks may be in the works, said Spiegel Online. FBI investigators were at the scene within hours of the attack and an investigation by the German authorities is under way. The suspect, who is believed to have worked at the airport and grown up in Germany, has been identified by the Kosovan government as Arif Uka, originally from the northern town of Mitrovica.