German federal prosecutors said Thursday that the attack is being investigated as a possible act of Islamic terrorism.
"The suspect is accused of killing two US military personnel and seriously injuring two others," federal prosecutors said in a statement. "Given the circumstances, there is a suspicion that the act was motivated by Islamism."
German police and American intelligence were investigating whether the 21-year-old man from Muslim-majority Kosovo had links to terrorist organizations or was acting alone.
Frankfurt police spokesman Manfred Fuellhardt said Aka got into an argument with airmen outside their military bus before opening fire, killing the bus driver and one other serviceman, and wounding two others, one of whom was in life-threatening condition. He said the attacker also briefly entered the bus.
The suspect then fled into the airport terminal, where he was quickly grabbed by two federal police officers and a US airman who had pursued him into the building, authorities said. He was disarmed without incident.
Fuellhardt told ABC that while being wrestled into submission, the suspect shouted either "Jihad Jihad" or "Allahu Akbar" – the Arabic phrase for "God is great".
According to ABC News, Rep. Patrick Meehan (R-Pa.) said at a House Homeland Security Committee hearing that the shooting looks like it could be a terrorist attack. Meehan said he was briefed by his staff, who are continuing to collect information.
US intelligence officials are running Uka's name through its terrorism data bases to see if he has come to their attention before, ABC News reported.
Kosovo Interior Minister Bajram Rexhepi told The Associated Press that German police have identified the gunman as a citizen of Kosovo.
"This is a devastating and a tragic event," Rexhepi said. "We are trying to find out was this something that was organized or what was the nature of the attack."
The uncle of the suspected shooter said he is a German Muslim who worked at Frankfurt airport.
US President Barack Obama made an unscheduled appearance to say, "I am saddened and I am outraged by this attack" and US investigators would work with German authorities and "spare no effort" to ensure that "all of the perpetrators are brought to justice."
He added that the killings were a "stark reminder of the extraordinary sacrifices" of American service members.
German Chancellor Angela Merkel condemned the "terrible incident."
"We'll do everything possible to find out what happened," she said.
AFP, AP contributed to the report
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