FRANKFURT, Germany — A man armed with a handgun attacked a bus carrying U.S. Air Force troops at Frankfurt airport Wednesday, killing two airmen 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. Family members in Kosovo described the suspect as a devout Muslim, who was born and raised in Germany and worked at the airport.

Obama upset by attack

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

"I'm saddened and I'm outraged by this attack," he said.

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, while one airman suffered light injuries and a second suffered serious wounds and was in life-threatening condition, 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.

Were to have been deployed 

The airmen were bound to Ramstein Air Base from where they were to have been deployed to support an overseas operation, the U.S. military said, without elaborating.

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.