The suspected mass graves of up to 1700 captured Iraqi soldiers killed by the Islamic State group (IS) have been found in the city of Tikrit.
The sites are near the former US Army base, Camp Speicher.
Iraqi forensic teams have begun to excavate 12 graves following the city's recent liberation from IS.
The June 2014 incident is notorious after IS posted videos and pictures of the execution of the mostly Shia soldiers on social media.
Survivors say the militants questioned the victims to identity those who were Shia before killing them.
The exhumations have began just days after Tikrit fell to a combination of the Iraqi army and Shia militias following a month long siege.
DNA testing will be used to identify the bodies once they have been exhumed, as many families have never had confirmation of their relatives' death.
"We dug up the first mass grave site today. Until now we found at least 20 bodies. Initial indications show indisputably that they were from the Speicher victims," Khalid al-Atbi told the Reuters news agency on Sunday.
"It was a heartbreaking scene. We couldn't prevent ourselves from breaking down in tears. What savage barbarian could kill 1,700 persons in cold blood?" he added.
The murder of the soldiers has become a lightening rod for Shia militias who have vowed to avenge the killings.
The militias have been credited with halting Islamic State's advance last year, but have themselves been accused of war crimes.
Some of the burial sites are in the presidential compound of the former Iraqi president Saddam Hussein, which became the IS headquarters after they took the city last year.
The offensive launched on 2 March to take back Tikrit involved some 30,000 fighters, two-thirds of them from the Popular Mobilisation (Hasid Shaabi) - a force comprising dozens of Iranian-backed Shia militia.
The Iraqi army is now expected to turn its attention to Mosul, 225km (335 miles) north along the Tigris river.
IS's most significant stronghold in Iraq, Mosul, presents a far greater challenge than Tikrit for the US-led air coalition and Iraqi ground forces.