Security officials in Iraq say they suspect that militants who killed about 50 army recruits on Saturday may have been given inside information.
The gunmen had information about the recruits, say officials
The officials say the militants must have been informed that the soldiers, who had just completed basic training, were being taken home in minibuses.
Their bodies were found close to their training camp, in the north-east of the country, near the Iranian border.
Local police say the convoy appeared to have been ambushed.
"There was probably collusion among the soldiers or other groups," Diyala province deputy Governor Aqil Hamid al-Adili told al-Arabiya television.
"Otherwise, the gunmen would not have got the information about the soldiers' departure from their training camp and that they were unarmed," he added.
"This was an execution. We found the dead lying face down by the roadside with a single bullet wound to the head," Iraqi national guard commander Ali al-Kaaki said, according to the AFP news agency.
The BBC Baghdad correspondent says the increasing level of attacks shows both the ruthlessness and growing capability of militant groups in Iraq.
The rebels who ambushed the army recruits close to Baquba were reportedly dressed in police uniforms.
Web site claim
A website often used by militants published a statement attributed to the group led by Jordanian militant Abu Musab al-Zarqawi in which it claimed to have carried out the army recruits massacre.
Al-Zarqawi's recently renamed group, the Al-Qaeda Organisation for
Holy War in Iraq, said they had killed "apostates".
"The mujahideen killed them all, stole two vehicles and the salaries they had just received from their masters," a statement said.
Its authenticity could not be verified.
"It appears that they were ambushed by a large, well-organised force with good intelligence," a senior Iraqi security source told Reuters news agency.
The BBC's Claire Marshall in Baghdad says the army recruits were believed to have just finished a training course at a camp near the Iranian border.
They were ambushed on a remote road in Diyala province at sunset on Saturday, while travelling in three minibuses, in civilian clothes.
Reports suggest that the recruits were mainly from Shia cities in the south of Iraq, including Basra, Amara, Kut and Nasiriya, and were returning there after training in the desert camp.
Correspondents say Iraq's fledgling security forces are a prime target for militant groups.
Earlier on Saturday, 20 newly-recruited members of the Iraqi security services were killed in separate suicide bomb attacks.