The bodies were found on Sunday near Nibaie, a few miles north of the capital Baghdad. Their hands were bound and they had been shot, a security official said.
They were part of a group of 35 police volunteers who went missing on Monday evening after being kidnapped by insurgents.
The men had gone from Samarra, 125km north of the capital, to Baghdad to join the police force, but had been turned down and were heading back home.
"Eight cars and minibuses loaded with dozens of gunmen held up the bus and took the men into the desert," according to security officials quoting one man who was shot and injured as he escaped and who arrived home on Wednesday.
US military spokesman Lieutenant-Colonel Frederick Wellman said the men decided to return home by commercial bus, adding that the bus was diverted off the main highway because of roadblocks put up on Monday around Mishahda after a US Apache attack helicopter was shot down with the loss of its two-man crew.
"They were then stopped by insurgents who boarded the bus and discovered some were would-be police recruits," Wellman said.
Other passengers were taken off the bus and the insurgents then drove away with the police volunteers in the bus, he added.