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