During interrogation Ibrahim said he fled to neighbouring Syria after the US-led invasion and returned to the country in February 2007.
A month ago, another Ba'ath party member had phoned him from Syria to carry out the attack in Baghdad, Ibrahim said.
"The truck bomb was prepared by other party members in Maqdadiyah, some 100 km northeast of Baghdad," he said. Ibrahim paid $10,000 to bring the explosives-filled trucks to Baghdad.
The truck bombs were detonated near the Iraqi finance and foreign ministries in central Baghdad Wednesday, killing 87 people and wounding more than 1,200 others.