The attack killed two airmen and wounded two.
Investigators were scrambling to find out more about the 21-year-old Kosovo citizen after they say he fired shots at U.S. military personnel on a bus outside Terminal 2 at one of Europe's busiest airports this afternoon.
Rep. Patrick Meehan, a member of the U.S. Homeland Security Committee, said it looked like a terrorist attack. The New York Times quoted a man whose office is near the site of the shooting who seemed to back up that assertion.
Speaking on condition of anonymity to protect his business, the man said witnesses told him that the gunman first talked to the military personnel to find out who they were and then opened fire, shouting "God is great" in Arabic.
The shooting occurred as the bus sat outside Terminal 2 at the airport, Frankfurt police spokesman Manfred Fuellhardt said, according to CNN.
Fuellhardt said the airmen had just arrived from England and had boarded the bus bound for the American military base at Ramstein, about 50 miles from the airport.
The suspect reportedly argued with some of the airmen before authorities say he opened fire, killing the bus driver and another serviceman, and seriously wounding another. Injuries to fourth airman were apparently not as serious, Fuellhardt said.
"Everything seems to have taken place inside the bus," police spokesman Jürgen Linker said, according to Deutsche-Welle, adding that evidence gathered so far indicates that the accused attacker was acting alone.
Police said the alleged shooter tried to flee the scene but was overpowered and arrested.
Füellhardt said that the dark blue-gray bus had regular German license plates, part of American military policy not to stand out. But he said the vehicle still may have looked different than other German buses, the Times reported.
President Barack Obama told reporters in Washington that he was "saddened and outraged" by the attack and called for those responsible to be brought to justice.
"We don't know the details, but I would like to express how shocked I am," German Chancellor Angela Merkel said at a news conference.
"We will do all we can to investigate what happened. It is a dreadful incident, and Germany will do all it can to bring clarity to this matter."
"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."
Spokesmen for the Pentagon and the Air Force in the U.S. had no immediate information on the incident.
The worst previous attack on U.S. servicemembers in Germany was in 1986, when a bomb was planted in a Berlin disco. Two soldiers died along with a Turkish woman, and 230 people were injured, including 44 Americans.
The U.S. blamed Libya and bombed Tripoli and Benghazi in retaliation.