German federal prosecutors said Friday they were investigating a Turkish national on suspicion of spying on dissidents for Turkey's secret services.
The suspect, identified as Ali D., was arrested in a Duesseldorf hotel on September 17 after an employee noticed a weapon on him, AFP quoted the prosecutors as saying in a statement.
They said there were indications Ali D. was collecting information on supporters "of the so-called Gulen movement" living in the Cologne area "in order to pass it on to the Turkish MIT intelligence service".
Fethullah Gulen is a longtime foe of Turkish President Recep Tayyip Erdogan.
Turkey accuses Gulen of masterminding a failed coup in 2016 that left hundreds dead and thousands more injured.
The exiled cleric, who lives in Pennsylvania, insists he is the head of a peaceful network of charities and companies, and denies any links to the failed putsch.
But Erdogan, who once was allied with Gulen, describes him as the leader of a "terrorist" group seeking to infiltrate and overthrow his government.
Since 2016, Turkey has arrested tens of thousands of people suspected to have links to Gulen.
Ali D., 40, also stands accused of a weapons violation after a police search of his hotel room turned up 200 rounds of ammunition, prosecutors said.
Local media reported that heavily armed officers from Germany's SEK special forces had stormed the hotel to arrest the suspect in what appeared to be a major police deployment, with an armored vehicle and a row of police vans pictured at the scene.
According to the Tagesspiegel newspaper, local authorities in North Rhine-Westphalia state, home to the cities of Duesseldorf and Cologne, believe it is possible an attack was being planned on Gulen supporters.
Federal prosecutors have taken over the investigation from the Duesseldorf prosecution office, as is standard procedure in cases of suspected foreign agent activity.
Since the failed 2016 coup, Turkey has "repatriated" dozens of people accused of belonging to Gulen's network, regardless of the repercussions such actions may cause abroad.
German officials have in recent years accused Ankara of using Turkey's MIT service or clerics to spy on suspected Gulen followers in Germany.
The spying allegations have added to already strained ties between the two NATO countries over a range of issues, including human rights concerns in the wake of the 2016 coup bid.
Germany is home to around three million ethnic Turks, the largest diaspora abroad and a legacy of the European power's "guest worker" program of the 1960s and 70s.
