FRANKFURT, Germany — A gunman opened fire on a bus at Frankfurt’s airport on Wednesday, killing two U.S. airmen and wounding two others, German authorities said.

Police suspect a 21-year-old Kosovo-born man was behind the attack on a military bus that was taking the airmen to Ramstein Air Force Base, apparently after some kind of dispute.

It was not immediately clear if the gunman’s motive was political. The assailant ran from the scene, but was overpowered and arrested by police in the nearby airport terminal.

Both German Chancellor Angela Merkel and U.S. President Barack Obama voiced shock.

The attacker shot three U.S. Air Force military policemen and the bus driver in the incident. About 12 military police who had just arrived on a commercial flight from London were on the bus.

U.S. European Command in Stuttgart confirmed the deaths but did not release the names of the victims, pending family notification.

Boris Rhein, interior minister of Hesse state, where the attack occurred, said the suspect first spoke to an airman standing outside the bus before shooting. He then shot the bus driver, Rhein told Germany’s Hessischer Rundfunk television.

There was no indication why the assailant was carrying a pistol. German gun laws license very few people to walk around armed.

But a German news website, Spiegel Online, said investigators presumed the gunman was deliberately seeking members of the U.S. armed forces as victims. It added that he had been carrying extra ammunition. The Frankfurt police would not confirm the report.

Over recent years, German police had foiled every one of several Islamist plots aimed against U.S. servicemen on German soil.

Rhein said the suspect had roots in Kosovo and lived in the Frankfurt area. He was interrogated for hours by police.

The bus passengers were on their way to deployments in Iraq and Afghanistan, with orders to fly from nearby Ramstein Air Force Base, the biggest U.S. military airfield in Europe, Air Force spokesman Wolfgang Hofmann said.

All were members of the Security Forces, the military police section of the Air Force, and had been based at Lakenheath, England.

At the scene, Rhein stressed that he was not using any words to suggest it was a terrorist attack and was calling the incident a homicide only, but added, “”At the moment, I don’t rule anything out.””