A German man has been charged with murder and committing a war crime for allegedly helping torture and kill prisoners as a member of the Islamic State group in Syria, German federal prosecutors said Thursday.

Prosecutors said the charges against the 27-year-old, who is already in prison for a previous conviction, were filed at the Duesseldorf state court earlier this month.

Nils D., whose surname wasn't provided in line with privacy rules, traveled to Syria in October in 2013 and returned to Germany about a year later. He was arrested in January 2015.

The suspect was sentenced in 2016 to 4 1/2 years in prison for membership in a foreign terrorist organization. He now faces a second trial for involvement in torturing prisoners at an IS prison in Manbij. Prosecutors say at least three prisoners died.

Separately, federal prosecutors announced the arrest of a 31-year-old German woman on suspicion of being a member of IS. Sabine Ulrike Sch., whose surname authorities also didn't divulge, was arrested Thursday in the Karlsruhe area.

She is alleged to have traveled to Syria in December 2013 where she married a high-ranking member of IS. The couple, who had at least two children, received $100 a month from the group, prosecutors said.

The suspect is alleged to have written a blog praising life in IS-controlled areas and declaring herself willing to commit a suicide attack. She also used a messaging service to promote IS.

Her husband was killed in late 2016, and Sch. was detained by Kurdish forces almost a year later along with other wives of IS members. She returned to Germany in April.