Authorities in Europe are calling the shooting at a Frankfurt, Germany, airport that claimed the lives of two U.S. servicemen an act of Islamic terrorism, though U.S. investigators said it is too soon to tell.

The suspected gunman, 21-year-old Arid Uka, who was captured immediately after Wednesday's shooting, admitted to the deadly attack and said he acted alone, German Interior Minister Boris Rhein said today, according to a report by The Associated Press. Uka, an ethnic Albanian from Kosovo who was described as a long-time resident of Germany, had been apparently radicalized over the last few weeks, Rhein said.

Uka allegedly shouted "Allahu Akbar" as he opened fire on a bus carrying U.S. airmen in Frankfurt, Germany, Wednesday, killing two and wounding two others before his gun jammed, officials said.

The U.S. has launched an FBI-led investigation into the shooting and officials told ABC News it is too soon to determine whether the attack was an act of terror and if the suspect acted alone. However, a senior U.S. intelligence official told ABC News the attack was likely terror-related.

Uka is another "dot in the matrix" of a rising threat of fundamentalist terror originating in the Balkans, the official said. Another man from Uka's home town in Kosovo that was among those arrested in Raleigh, North Carolina, on terrorism charges in July 2009.

While airport security in Germany has been heightened following the shooting, the U.S. will also examine whether security for troops in transit is satisfactory, officials said.

Obama 'Saddened and Outraged'

President Obama made an unscheduled appearance before reporters Wednesday to say he was "saddened and I am outraged by this attack" and that 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 service members.

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.

In the attack, Uka allegedly fired nine times, killing 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 service members 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 Afghanistan to support Operation Enduring Freedom, according to a statement posted on the Ramstein Air Force Base website.

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.

Who Was Gunman at Frankfurt Airport?

The gunman was identified 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.

The names of the deceased are being withheld until 24 hours after notification of next of kin.

ABC News' Matthew Cole, Avni Patel, Huma Khan and Lee Ferran contributed to this report.