German prosecutors say they've indicted a 21-year-old woman on suspicion of membership in the Islamic State group and of keeping three Yazidis as slaves in Syria.

Federal prosecutors said Wednesday that the German-Algerian woman, identified as Sarah O. for privacy reasons, traveled to Syria as a teenager in 2013, joined IS and married a fellow German IS recruit.

Both allegedly received firearms training and conducted "guard and police duties" in IS-controlled areas. They also forced a Yazidi girl and two Yazidi women to work in their household and convert to Islam.

She was arrested in September upon her return to Germany.

O.'s parents-in-law, 51-year-old Ahmed S. and 48-year-old Perihan S., allegedly helped their sons supply IS with equipment such as firearms magazines and scopes. They are indicted on suspicion of aiding IS.