Islamic State group released a 22-minute video showing its fighters massacring Iraqi soldiers in 2014.
The events occurred at Camp Speicher, near Tikrit, Iraq.
In one scene, a child about 12 years old appears to execute two men by shooting them in the head.
Other images seem to show several men being shot with machine guns in mass graves.
“This is a message I address to the whole world and especially to the Rafidha dogs, I tell them we are coming,” an Islamic State group fighter says on the video, using a derogatory term for Shia Muslims.
The incident occurred last summer as Islamic State group fighters overran large areas of Iraq, including Tikrit, while government troops fled. The victims of the massacre had been trying to escape from the army base when they were captured.
CBS News reported that captured soldiers were herded on to large trucks, before being shot as they lay in shallow ditches.
At least 600 bodies have been recovered from graves, Daily Mail reported. The death toll could be much higher, because Islamic State group militants threw many of the victims in the Tigris river.
Last week, a Baghdad court sentenced 24 alleged participants in the massacre to death, and released four due to lack of evidence. The defendants argued the confessions on which their trial was based had been extracted through torture.
By contrast, spokesman for the Iraqi Supreme Judicial Council Abdul Sattar Bayrkdar said before the beginning of proceedings that “the trial will be transparent and fair.”
Daily Mail reported that the trial lasted a matter of hours.
A further 604 suspects in the massacre are still at large, reports CBS.
“Justice is done,” Ali Abdul-Hamza, whose brother was a victim in the killings, told CBS. “We are relieved to see these criminals receiving the maximum punishment.”
Iraqi soldiers retook Tikrit earlier this year. According to Daily Mail, one of the sites of the massacre has been turned in to a shrine and has been visited by relatives.