Iraq's parliament was expected to meet in a special session on Friday morning to express condemnation at a suicide bombing that tore through a cafe in the building the day before, killing and wounded dozens.
The US military said eight people were killed and more than 20 wounded when a suicide bomber slipped through multiple checkpoints and blew himself up amid lawmakers having lunch.
It was the worst breach of security in the heavily fortified Green Zone that houses parliament, government offices and the US embassy.
The compound was set up after the US-led invasion of Iraqi in 2003.
An aide to parliament Speaker Mahmoud Mashhadani said the session would start at 11 am and was expected to condemn the blast.
The attack defied a two-month-old US-Iraqi crackdown in the city that has put tens of thousands of troops on the street to curb sectarian violence between majority Shi'ites and minority Sunni Arabs who were dominant under Saddam Hussein.
The operation has succeeded in reducing the number of targeted killings, but US and Iraqi commanders have found car and suicide bombers much harder to stop.
Iraq's authorities were investigating on Friday how explosives were smuggled into one of the most secure areas of the Green Zone.
The explosives would have had to pass through an outer checkpoint manned by US and Iraqi troops and multiple inner checkpoints guarded by security contractors and foreign troops that are part of the US-led coalition.
Iraqi military spokesman Brigadier General Qassim Moussawi, said it appeared to have been a well-planned operation that had taken advantage of "a deficiency in one of the security points". A review of procedures would need to be carried out, he said.
That would include looking at some VIPs and their bodyguards whose special identity cards allowed them to pass into the parliament, housed in a former conference centre, without being searched.
Entry into the conference centre is restricted to accredited parliamentary staff, lawmakers, security guards and journalists.
Access to the cafeteria itself is restricted to lawmakers, police and kitchen staff.
Two Shi'ite lawmakers said the metal detector used at the VIP entrance was working, but a Sunni legislator said when he arrived there was a power cut and bags were being manually searched. An agency cameraman said the scanner at a second entrance used by staff and journalists was operating.
Militants have rarely managed to carry out attacks inside the zone, although the sprawling area has come under increasing rocket and mortar attack in recent weeks.