A mass grave in an area recently re-taken from ISIS militants in Hamam al-Alil, Iraq.

:: Iraqi military investigators said Friday they have discovered two mass graves near a former ISIS group prison outside Mosul that contain the bodies of 500 ISIS victims.

The Media Cell Security Investigation team said in a statement that one grave near the Badoush Prison site contained the bodies of 470 prisoners killed by ISIS. It said a second grave contained 30 victims.

A security official who spoke on condition of anonymity said that based on records of prisoners who were at Badoush most are believed to have been Shiite Muslims or other minorities. The ISIS group is a Sunni extremist group.

Authorities were continuing to look for more graves.

A massacre at Badoush in June 2014 left 600 male inmates dead. A patch of scraped earth and tire tracks show the likely killing site, according to exclusive photos obtained by the imagery intelligence firm AllSource Analysis.

The ISIS group has scattered mass graves across Iraq and Syria.

The Associated Press last year documented and mapped 72 of them. For at least 16 of the Iraqi graves, officials do not even guess the number of dead.

In others, the estimates are based on memories of traumatized survivors, ISIS propaganda and what can be gleaned from a cursory look at the earth. But even the known numbers of victims ranges from 5,200 to more than 15,000.

Also on Friday, the spiritual leader of Iraq’s Shiite majority called on doctors from across Iraq to help civilians fleeing clashes in the latest fight against the ISIS group in Iraq.

Grand Ayatollah Ali al-Sistani called on medical workers should travel to areas around the battle for Tal Afar to help “to treat the wounded and treat them as a humanitarian, national and religious duty.”

The call during the Friday sermon delivered by Sistani’s representative Sheikh Abdulmehdi al-Karbalai, from the holy city of Karbala comes after the operation to retake Tal Afar began last week.

Tal Afar sits west of Mosul, where victory was declared against ISIS in July. Thousands of civilians are estimated to have been killed in the nine-month fight.

Iraqi defense officials say about 10,000 civilians remain inside the city.