The bodies of 36 men have been found dumped in a shallow river near the Iraqi town of Kut, police say.
The men - aged between 25 and 35 - were found at Areda, some 50km (31 miles) west of Kut, a spokesman told the BBC.
They were partially clothed, had been handcuffed and all had been shot in the head in an execution style.
Violence has increased in Iraq amid growing uncertainty over the future of a new constitution. Talks on a new charter are to continue on Friday.
Each of the 36 Iraqis had a 9mm bullet in the head, the Iraqi police spokesman said.
The state of the bodies indicated to the local police that they had been killed four or five days ago.
Six of them had metal handcuffs on and the other 30 plastic handcuffs.
Killings have become common in Iraq, fuelling tension between Sunni and Shias, correspondents say.
Early this month, at least 19 bodies were discovered near a school in south-western Baghdad.
Some of the dead had been blindfolded and shot, while others were beheaded, according to reports. All were men.
Within the space of three days on May, Iraqi police found the bodies of more than 50 people who appeared to have been killed in an execution style at different locations around Baghdad.