Iraqi forensic teams have begun excavating 12 suspected mass grave sites in the northern city of Tikrit.
The graves are thought to hold the corpses of as many as 1,700 soldiers killed last summer by so-called Islamic State militants.
The graves of the Shia soldiers are near the former US base of Camp Speicher.
The killings have become a symbol of the brutality of IS fighters and their hatred for Iraq's Shia majority.
Islamic State fighters have also attacked ethnic and religious minorities as well as fellow Sunni Muslims opposing them.
Images of Shia soldiers being machine-gunned in their hundreds, posted online by the jihadists, could rank as the deadliest single act of bloodshed during a decade of periodic sectarian war in Iraq.
The exhumation of burial sites on the late dictator Saddam Hussein's presidential compound came days after IS militants were driven from the city by Iraqi forces and Shia paramilitaries.
"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," Iraqi health official Khalid al-Atbi said yesterday.
"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 asked.
The victims' families have wondered for months about the fate of their friends and loved ones.
"The only positive thing is the victory in Tikrit," said Ali Hamad, whose cousin Kamal went missing at Speicher.
"We are happy. At least the families will soon know the fate of their sons and cousins," he said.
