Iraqi officials announced Monday the discovery of several mass graves near the city of Tikrit, state news media reported.
Officials with the Ministerial Council said in a statement that 10 mass graves had been found in the city, about 80 miles north of Baghdad, after a combined force of Iraqi troops and militias recently routed
"Eight graves were found inside the presidential palaces in Tikrit and two outside that area," Haidar Majid, a spokesman with the operations room of the council's general secretariat, told the Sumariya news outlet.
It was not immediately established who was responsible for the killings and how many bodies were found.
Islamic State fighters seized Tikrit in June as they swept through northern and western Iraq from their previous bases in Syria. They also took a former U.S. base nearby, once called Camp Speicher, capturing an estimated 1,700 Iraqi military cadets stationed there, most of them Shiite Muslims.
Later, the radical Sunni movement released images and videos showing terrified men stripped to the waist, led in a line as masked militants jeer at them. The men were then shown being executed and their bodies thrown into trenches.
Families of the cadets, many of whom have not been found or their remains recovered, mounted a protest in the capital, Baghdad, on Monday, according to local news outlets. They demanded that the government do more to account for the missing men.
The images of the slayings, which became known as the Speicher massacre, galvanized Shiite militias to join the fight against Islamic State. They formed the backbone of the pro-government's forces in the recent successful offensive to retake Tikrit. Some militia leaders vowed vengeance against Islamic State and those civilians who collaborated with its fighters in perpetrating any massacres.
The threats have fueled fear of sectarian violence reminiscent of the vicious chaos unleashed in 2006 and 2007, when Shiites and Sunnis engaged in reprisal killings against each other in the wake of the U.S. invasion of Iraq.
Since the retaking of Tikrit, pro-government forces have claimed to have found dozens of mass graves in the city and surrounding villages. The removal of bodies for analysis, however, has been hampered by the presence of explosive devices.
"Daesh keeps a bit of the grave open so we would approach and then be killed with an explosion," Jaafar Husseini, spokesman for the Hezbollah Brigades militia operating near Tikrit, said in an interview last week, referring to Islamic State by its Arabic acronym.
Majed Aaraji, head of Iraq's central investigative court, said last week that 590 arrest warrants had been issued seeking those accused of carrying out massacres, though the names were not immediately released, Sumariya reported.