More than 150 people were killed in the suicide attacks
Iraq has arrested more than 60 security force members, including 11 senior officers over Sunday's twin suicide bombing in the capital, Baghdad.
Those arrested include the commanders of 15 checkpoints near to where the attacks took place.
The attack, in which more than 150 people were killed and 500 injured, was the deadliest in Baghdad for two years.
Correspondents say the scale of the bombings raised new questions over the competence of Iraqi security forces.
The BBC's Gabriel Gatehouse in Baghdad says it is not clear whether those arrested are accused of negligence or collusion.
However, he adds, it seems to confirm what many people have suspected - that the security forces are susceptible to infiltration by insurgents or are just not up to the job.
Iraqi Foreign Minister Hoshyar Zebari said in a BBC interview that he wanted the UN to investigate external interference, accusing Syria of providing a safe haven for the bombers, which Damascus has denied.
Mr Zebari asserted that security aims would have to be met for the planned US withdrawal to proceed according to plan.
"I think we are committed to the timetable according to the agreement. But definitely we need to maintain a reasonable degree of security and stability," he said. "Otherwise it may impact the withdrawal plans; they could be modified."
He added: "The Americans cannot just wash their hands to say 'we are no longer engaged or interested because we have our own timetable'."
It is feared that attacks will become more frequent as militant groups attempt to destabilise Iraq in the run-up to January's general election.
Meanwhile Iraqi MPs remain deadlocked over the rules governing the election.
Efforts to pass a bill failed again on Thursday as too few MPs were present to vote.
Kurdish MPs boycotted the session over a dispute about voter registration, meaning the quorum was not met.
Differences between the Arab and Kurdish factions centre on the disputed northern oil city of Kirkuk.
Tens of thousands of Kurds were displaced from the city under Saddam Hussein's rule and there is a heated debate about whether those who have returned since should be able to vote.
Parliamentarians have until 31 October to break the stalemate if the election is to go ahead on time.