BERLIN (Reuters) - Germany’s chief prosecutor has charged a 28-year-old German citizen with murder, belonging to a terrorist group and committing war crimes for his role in the execution of six detainees by Islamic State militants in Syria in 2015.

The man, identified as Harry S. in court documents, has been in custody since returning to Germany in July 2015, shortly after prosecutors said he took part in the executions in the ancient Syrian city of Palmyra.

A spokeswoman for the Federal Prosecutor’s Office in Karlsruhe said on Tuesday the man had traveled to Syria in early April 2015 to join Islamic State.

“In mid-June 2015 members of Islamic State publicly shot dead six detainees at the market place in the Syrian city of Palmyra,” the statement said. “The accused, who was armed with a pistol, personally took one of the detainees to the execution spot and prevented the others from escaping.”

Last year the man was sentenced to three years in jail after being found guilty by a regional court of belonging to a foreign terrorist organization and breaching laws on weapons of war.

An estimated 820 people have left Germany to fight alongside militants in Syria and other war zones in the Middle East.