Human Rights Watch says new evidence indicates Islamic State (IS) fighters have killed between 560 and 770 men captured when it overran the city of Tikrit, north of Baghdad.
IS insurgents captured Tikrit in June, during an offensive in which it seized large swaths of northern and western Iraq.
The New York-based watchdog said on September 2 the new toll is three times higher than what was initially reported, and that the number of victims may well be even higher as more evidence emerges.
It added that all or most of those killed in Tikrit were "apparently" captured Iraqi army soldiers.
On September 2, dozens of family members of the missing soldiers stormed into parliament in Baghdad in an effort to pressure authorities for word on their relatives’ fate.