Between nine and 12 armed men were able to trick their way past Iraqi soldiers guarding the Provisional Joint Co-ordination Centre because they spoke perfect English, were wearing new US military fatigues and were armed with American weapons.
They were also travelling in black GMC Suburban vehicles, the type used by US government convoys.
The attackers then stormed into the compound, threw stun grenades at a group inside a police office and shot dead an American soldier, the US military said.
They then fled with four US troops who had been attending a meeting to plan security measures for the 10-day Shia mourning ceremony of Ashura, which culminates on January 30.
|"The precision of the attack, the equipment used and the possible use of explosives to destroy the military vehicles in the compound suggests that the attack was well rehearsed prior to execution" |
Lieutenant-Colonel Scott Bleichwehl, US military spokesman
Iraqi police, who had become suspicious when the convoy failed stop at a roadblock, chased the vehicles and found them abandoned near the town of Al-Mahawil in Babil province.
"Two soldiers were found handcuffed together in the back of one of the SUVs [sport utility vehicles]. Both had suffered gunshot wounds and were dead. A third soldier was found shot and dead on the ground," a military statement said.
"Nearby, the fourth soldier was still alive, despite a gunshot wound to the head."
He was taken to a nearby hospital but died on the way.
"The precision of the attack, the equipment used and the possible use of explosives to destroy the military vehicles in the compound suggests that the attack was well rehearsed prior to execution," Lieutenant-Colonel Scott Bleichwehl, the US military spokesman, said in the statement.
"The attackers went straight to where Americans were located in the provincial government facility, bypassing the Iraqi police in the compound."
The military said it was "looking at all the evidence to determine who or what was responsible for the breakdown in security at the compound and the perpetration of the assault".
Three days after the killings, the US military in Baghdad announced the arrest of four suspects over the attack and said they were detained after a tip from a Karbala resident. No further information was released about the suspects.