Investigators of the German Federal Criminal Office have received information that the gunman, a 21-year-old man from Kosovo who has been living in Frankfurt for many years, had contacts with people associated with radical Muslim groups, media reports have said.
Two members of the United States Air Force were killed and two others injured in the incident. According to one report, the gunman had linked up his Facebook profile with websites propagating radical views.
There are also growing indications that the attacker was well informed about the travel plans of the US soldiers, the reports said. It was possible that he had planned the attacker for a long time and he may have accomplices, said authorities.
Boris Rhein, interior minister of the state of Hessen where Frankfurt airport is located, described the attack as a 'hideous crime' which must be cleared up in cooperation with the American security authorities.
Nearly a dozen soldiers belonging to the military police of the US Air Force, who are stationed at the Lakenheath air base in Britain, arrived at the Frankfurt airport on Wednesday afternoon from London on board a civilian flight.
They boarded the military bus parked outside Terminal 2 to travel to the US air base in Ramstein. From there, they were scheduled to leave for Afghanistan or Iraq. Ramstein has been a hub for military operations in the two countries.
According to the police, the attacker fired several shots at the bus, killing a soldier and the driver, who were standing near the bus.
He then entered the bus and fired at the soldiers sitting inside. He was overpowered and arrested by the police when he tried to escape by running inside the terminal. Besides the local police, the Federal Criminal Office, Frankfurt State Prosecutor and American security authorities are involved in the investigations.
The Central Intelligence Agency and the Federal Bureau of Investigation are sending a large team of specialists to Frankfurt. German Chancellor Angela Merkel and US President Barack Obama have sharply condemned the attack.
The German government will do everything to clear up this 'shocking incident', she said. The Chancellor also expressed her condolences for the bereaved families of the soldiers.
President Obama said the US government would do everything to find out how this "dastardly act" happened.
Flight operations at the Frankfurt airport were not affected by the attack and most of the air travellers were unaware of it, an airport spokesman said.
Germany had stepped up security for airports, railway stations and other public places at the end of last year following warnings that Islamic terrorists were planning to carry out attacks on high-profile targets in this country.
Image: A police car stands next to the US Army bus that was attacked by a gunman in front of Frankfurt airport | Photograph: Ralph Orlowski/Reuters