Federal prosecutors are investigating a espionage case involving a Turkish man suspected of allegedly tracking people he believed to be related to Fethullah Gulen, the late rival of Turkish President Recep Tayyip Erdogan.

According to the prosecutor’s office, charges have been brought against Mehmet K., who is believed to have sent information to Turkish police and intelligence agencies in anonymous letters.

However, the suspect is not in custody, the Federal Prosecutor’s Office clarified.