FRANKFURT, Germany >> A man armed with a handgun attacked a bus carrying U.S. airmen outside Frankfurt airport Wednesday, killing two Americans and wounding two others before being taken into custody, authorities said.

Boris Rhein, the top security official in the German state of Hesse where the shooting took place, identified the shooter as a 21-year-old from Kosovo.

In Washington, President Barack Obama promised to “spare no effort” in investigating the slayings.

The attack came as the bus sat outside the airport’s Terminal 2, according to Frankfurt police spokesman Manfred Fuellhardt. The bus driver and a passenger were killed, one person suffered serious wounds and another light injuries, he said.

The attacker and U.S. military personnel apparently had an altercation in front of the bus just before the man started shooting, Fuellhardt said. The attacker also briefly entered the bus, and was apprehended by police when he tried to escape.

The U.S. has drastically reduced its forces in Germany over the last decade, but still has some 50,000 troops stationed here. It operates several major facilities in the Frankfurt region, including the Ramstein Air Base, which is often used as a logistical hub for operations in Afghanistan and Iraq.

U.S. Air Force Europe spokeswoman Maj. Beverly Mock said all four victims were airmen. They were based in Britain, a U.S. Air Force spokesman for the Lakenheath airfield in eastern England said.

Lakenheath is home to the 48th Fighter Wing, the only F-15 fighter wing in Europe. It employs some 4,500 active-duty military members, as well as 2,000 British and U.S. civilians.

In Berlin, German Chancellor Angela Merkel expressed sympathy for the victims and their families and pledged that Germany would do everything in its power to investigate the crime.

“It is a terrible event,” she said.

The German news agency DAPD quoted Rhein, the security minister who rushed to the scene of the shooting, as saying there were no indications of a terrorist attack.

Still, a member of the U.S. House Homeland Security Committee, Rep. Patrick Meehan, said in Washington that it looked like a terrorist attack. The chairman of the subcommittee that focuses on terrorism and intelligence added he did not have all the facts yet and was still being briefed.

At Frankfurt airport, taxi cab driver Salimi Seraidon was sitting at a stand about 200 yards (meters) away when the attack took place and said it was over quickly as police rushed to the scene.

“We just heard the shots,” he said.

Kosovo Interior Minister Bajram Rexhepi identified the suspect as Arif Uka, a Kosovo citizen from the northern town of Mitrovica.

“This is a devastating and a tragic event,” Rexhepi said. “We are trying to find out whether this was something that was organized or what was the nature of the attack.”

The bus was transporting a Security Forces team assigned to RAF Lakenheath, U.K., from the airport to Ramstein. They were on their way to support overseas military operations.

Kosovo remained part of Serbia amid the collapse of former Yugoslavia in the early 1990s, but a struggle for independence by ethnic Albanians there eventually led to the Kosovo war in 1998. The bloodshed was halted only in 1999 when NATO stepped in and bombed Serbia, followed by the deployment of peacekeepers. The NATO-led Kosovo Force (KFOR) still has some 8,700 troops there provided by 32 nations, including the U.S. and Germany.

The northern town of Mitrovca is best known for the ongoing ethnic division between majority ethnic Albanians and minority Serbs. The former mining town, however, has also been the focus of reports that it breeds radical Islamic extremists.

In the past Western intelligence reports have said the region could be an ideal recruitment ground for the so-called “white al-Qaida” — Muslims with Western features who could easily blend into European or U.S. cities and execute terrorist attacks.

The American forces in Germany have been the target of previous terror attacks, including a 1986 bombing at a disco in then-West Berlin that was frequented by U.S. servicemen. Two soldiers and one civilian were killed and 230 others injured in that attack, which a Berlin court in 2001 ruled was organized by the Libyan secret service and aided by the Libyan Embassy in then-communist East Berlin.

A leftist terror group, the Red Army Faction, was also responsible for a string of attacks on Americans in the 1970s and 1980s before the group was disbanded in 1998.

More recently, German police thwarted a plot in 2007 to attack U.S. facilities by members of the extremist Islamic Jihad Union. Four men had planning to attack American soldiers and citizens at facilities including the U.S. Air Force’s Ramstein Air Base in Germany but were caught before they could carry out the plot.