The bodies of 19 Iraqis have been found at a football stadium in Haditha, north of the capital Baghdad.
Eyewitness said they appeared to have been lined up against a wall and shot.
The dead wore civilian clothes. First reports said they were soldiers, but the defence ministry later said they were fishermen from the south of Iraq.
Earlier, a suicide car bomb attack aimed at a US patrol in Baghdad killed at least two Iraqi civilians, one of them a child.
A further five people were injured in the blast.
In another attack, on a police station in Baghdad's southern district of al-Doura, a bomb exploded in a parking area, damaging 20 vehicles.
Shortly afterwards, a nearby police patrol was the target of an explosion that injured three civilians.
Late on Tuesday, two US soldiers were killed and four wounded by a bomb near the road to Baghdad airport.