A gunman shouting "Allahu Akbar" opened fire on a bus carrying U.S. airmen in Frankfurt, Germany, killing two and wounding two others before his gun jammed and he was subdued, officials said.
An ethnic Albanian from Kosovo was taken into custody and the FBI was heading an investigation because U.S. citizens were killed and to determine whether the shooting was an act of terrorism.
President Obama made an unscheduled appearance to say, "I am saddened and I am outraged by this attack" and U.S. 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 servicemembers.
Sources told ABC News that the victims were on a bus at Frankfurt airport. The bus was marked United States Air Force and was carrying 13 or 14 people, plus the driver. U.S. intelligence is trying to determine whether the shooting occurred while the gunman was on the bus or while he was trying to board the bus.
When he opened fire, the gunman, identified as a long-term resident of Germany, shouted "Allahu Akbar," according to sources. He fired nine times, killling two and critically wounding two others before the gun jammed and he was subdued by other passengers. While being wrestled into submission, the suspect shouted either "Jihad Jihad" or "Allahu Akbar," sources said.
One of the dead was the bus driver, military officials said.
The servicemembers who were attacked were members of a Security Forces team assigned to RAF Lakenheath in Great Britain. They were being transported to Ramstein Airbase and were en route to support Overseas Contingency Operations, although their exact destination was not stated.
Rep. Patrick Meehan, R-Pa., said at a House Homeland Security Committee hearing today 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.
The gunman was identifed by sources to ABC News as Arid Uka, although other spellings give his name as Arif Uka. Although he has lived in Germany for years, he is a citizen of Kosovo and his family is from the northern town of Mitrovica.
U.S. intelligence officials are running Uka's name through its terrorism data bases to see if he has come to their attention before.
Who Was Gunman at Frankfurt Airport?
"This is a devastating and a tragic event," Kosovo Interior Minister Bajram Rexhepi said. "We are trying to find out was this something that was organized or what was the nature of the attack."
The names of the deceased are being withheld until 24 hours after notification of next of kin.
ABC News' Huma Khan contributed to this report.