FRANKFURT—The suspected gunman who killed two U.S. servicemen at Frankfurt airport Wednesday had links to Germany's radical Islamist circles, but German investigators say they currently believe he carried out the attack on his own.

Investigators suspect Arid Uka, a 21-year-old ethnic Albanian whose family moved to Germany from Kosovo, had an "Islamist" motivation for firing at U.S. Air Force personnel on a bus at the airport, according to the office of Germany's chief federal prosecutor, Monika Harms.

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