The bus was taking them from a British-run police academy in a town 12km from the southern city Basra on Sunday.
A police captain in Basra said: "The men were forced off a bus on the city's outskirts at about 4:00pm (1300 GMT)."
The police then discovered the bodies dumped in several locations around the city about four hours later.
Meanwhile, at least 17 fighters died after US forces fought off a series of attacks in Balad, which lies about 80km north of Baghdad.
The US military said in a statement that soldiers called in an air strike after the first ambush, which left four attackers dead. They killed 13 more after fighting off the second assault with air and ground fire.
"The terrorists were planning to ambush the coalition ground force. The plan did not succeed. No coalition forces were injured during the attack," the statement said.
During the fighting, US soldiers spotted secondary explosions, suggesting that roadside bombs had been laid for them but had detonated prematurely, the statement said.
Journalist shot dead
In other incidents, two police officers were shot dead in an ambush by unidentified assailants, while a popular Iraqi state television presenter was shot dead along with her driver, after being kidnapped in Baghdad.
Journalists at the network told AFP that Nakshin Hamid, an Iraqi Kurd, worked for programming that was broadcast to Iraq's Kurdish and Christian minorities on Al-Iraqiya television and that she had recently been forced to move house after receiving death threats.
Abbas Aboud, assistant news editor, said:"She was kidnapped this morning along with her colleague, the driver, as she came in to work. Less than 30 minutes later someone called us from her mobile and said: 'We killed her'."
A security official said that the 31-year-old's body was found with the body of her driver, Anas Qassem Najm.
In another violent incident, armed men opened fire on a bus carrying pilgrims back from Makka as it neared the town of Khalis, north of Baghdad, wounding four travellers and killing their imam, Sheikh Ghazi al-Dulaimi from Baquba, police said.
In nearby Baquba, a gang broke into the home of an 80-year-old woman, gunned her down, and kidnapped her sons, according to medical sources.
Two Iraqi army soldiers were also killed by a roadside bomb.
Meanwhile, thousands of US troops were scouring Baghdad for a missing soldier. An American of Iraqi descent, it is believed that he was kidnapped when he slipped out of the fortified Green Zone a week ago to visit relatives.
A relative of the missing soldier told AFP that he was kidnapped by masked armed men from a family home after coming to see his secret Iraqi wife.
Speaking on condition of anonymity, he said: "The family was against the marriage, it was thought she had bad morals."
US forces fear the captive may be held in Sadr City and have set up checkpoints around the area.