FRANKFURT, Germany (AP) — The attack on a busload of U.S. Air Force troops at Frankfurt airport that killed two is being investigated as a possible act of Islamic terrorism, German federal prosecutors said Thursday.

Two airmen were also wounded late Wednesday when a man identified as a 21-year-old ethnic Albanian from Kosovo fired on the servicemen at close range. His family said the young man worked at Frankfurt airport and was a devout Muslim.

“The suspect is accused of killing two U.S. military personnel and seriously injuring two others,” federal prosecutors said in a statement. “Given the circumstances, there is a suspicion that the act was motivated by Islamism.”

Federal prosecutors said they had taken over the investigation of the Wednesday afternoon shooting, and are working on conjunction with Frankfurt and federal police, as well as American authorities.

The suspect was taken into custody immediately after the shooting and is to appear later Thursday in federal court.

Frankfurt police spokesman Juergen Linker told the DAPD news agency that one airman remained in critical condition after being shot in the head. The other wounded airman was not in life-threatening condition, Linker said. Both men were being treated at the Frankfurt University clinic. None of the victims have yet been identified, pending notification of next of kin.

The attacker’s family in northern Kosovo identified him as Arid Uka, whose family has been living in Germany for 40 years. At his father’s home in Frankfurt on Thursday, a man yelled at reporters to “go away,” threatening to call police.

Kosovo is mostly Muslim, but its estimated 2 million ethnic Albanians are strongly pro-American due to the U.S.’s leading role in NATO’s 1999 bombing of Serb forces that paved the way for Kosovo to secede from Serbia.

The U.S. Embassy in Kosovo’s capital Pristina said in a statement that “the act of a single individual will in no way affect the deep and abiding friendship between our two countries.”

The suspect’s uncle, Rexhep Uka, said the suspect’s grandfather was a religious leader at a mosque in a village near Mitrovica, and that Arid Uka was a devout Muslim himself.

But he said the family was pro-American and was also having a hard time imagining that their nephew was involved.

“I love the Americans because they helped us a lot in times of trouble,” he told The Associated Press in Kosovo. “I had an American neighbor and we never had a problem. What happened in Germany is beyond me.”

Behxhet Uka, a cousin of the suspect, said he had spoken to the gunman’s father Frankfurt by telephone several times. The family told him that they only knew that their son did not come home from work at the Frankfurt airport on Wednesday.

“We heard about this from the local police, and it was confirmed that this shooter was my first cousin,” he said. “I would hope that this is not true, but if it is true, it will be very hard for us here in Kosovo. We could not imagine something like this would happen because Americans are our brothers.”

Frankfurt airport spokesman Alfred Schmoeger said he had “no information” about Uka working at the airport, but said it was being checked.

“We have 70,000 people who work here at 500 businesses,” he said.

Police said the attacker had an altercation with U.S. military personnel in front of a bus outside the airport’s Terminal 2. They said the man started shooting, then boarded the bus briefly and was apprehended by police when he tried to escape.

The airmen were based in Britain, a U.S. Air Force spokesman for the Lakenheath airfield in eastern England said. They 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 some 50,000 troops stationed in Germany. 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.

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.

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Tomislav Skaro contributed to this report from Frankfurt; Eddy and Rising reported from Berlin.