The Istanbul prosecutor's office is charging Amnesty International's Turkey chief and 10 others for allegedly belonging to and aiding terror groups with the completion of an indictment Sunday, according to the country's official news agency.

The Anadolu news agency said officials concluded an investigation into the 11 human rights activists, who are now awaiting trial. The 17-page indictment was prepared by the prosecutor's office's terror crimes branch.

Police detained 10 human rights activists during a workshop on digital security at a hotel near Istanbul in July. Eight people, including Amnesty International's director in Turkey Idil Eser, German Peter Steudtner and Swede Ali Gharavi, were arrested. Two others were detained but released pending trial.

They are accused of aiding armed terror organizations in alleged communications with suspects linked to Kurdish and left-wing militants as well as the movement led by U.S.-based Muslim cleric Fethullah Gulen, who is accused of orchestrating last year's coup attempt. Gulen has denied the accusations.

The indictment includes a "secret witness" who claimed the activists were engaged in illegal activities during the workshop, according to Anadolu.

The case of Amnesty International chairman Taner Kilic, a lawyer imprisoned in June after his arrest in the western province of Izmir, has also been added on to the indictment. He was accused of using an encrypted mobile messaging application, allegedly used by Gulen's network.

The arrest of Steudtner and other Germans has been a cause of serious friction between Berlin and Ankara, and German Foreign Minister Sigmar Gabriel said the announcement of charges was a matter of "great concern."

He called the accusations against Steudtner "absolutely incomprehensible" and the threat of 15 years in prison for him "unacceptable," saying that his office had immediately contacted the Turkish government.

"We continue to do everything we can to bring the imprisoned German citizens, including Peter Steudtner, back to Germany," Gabriel said in a statement.