The police confirmed to Der Spiegel magazine that a probe was opened in August against unidentified persons on suspicion of causing grievous bodily harm.
At least two US officials working in Germany were reported in August to have developed nausea, severe headaches, ear pain, fatigue and insomnia over several months.
The Wall Street Journal said that cases of so-called Havana syndrome were also observed in Americans stationed in other European countries.
The condition was first reported by Americans posted to Havana in Cuba in 2016, earning the syndrome its name. It has since been rumoured to affect US officials in China, Russia, Austria and even Washington, DC.