Syrian activists say the Islamic State group has killed 12 people it held captive in the ancient city of Palmyra by shooting and beheading them, The Associated Press reports.

The Britain-based Syrian Observatory for Human Rights says four of the victims, teachers and government employees, were beheaded. The Observatory and the activist Palmyra Monitor network say four opposition fighter and four pro-government troops were first shot, then beheaded. The activists say the killings took place on Wednesday, January 18.

Some of the killings were carried out in the 2nd century Roman amphitheater, while others were said to have taken place in the courtyard of the Palmyra museum or in a former Russian base in Palmyra.

IS militants recaptured Palmyra in December, nine months after they were expelled from the city in a Russian-backed Syrian government offensive.