U.S. federal prosecutors have filed formal murder charges against a Kosovo man who allegedly shot dead two U.S. Air Force personnel in Germany in March.

The U.S. attorney's office in New York says Arid Uka chanted “God is great” in Arabic and kept shooting until his gun would not fire any more.

Uka is in custody in Germany. It is unclear if he will be extradited to the United States for trial.

Uka was a contract worker at Frankfurt airport when he stepped aboard a bus whose passengers included members of the U.S. Air Force.

U.S. officials believe he acted alone and was not a part of any terrorist group.