BAGHDAD  As Iraqi officials work to assign blame for the deadly attacks on the heart of the government on Sunday, concern is rising that a greater security threat may come from within the system in the form of corruption, from the top leadership of ministries down to soldiers who man checkpoints.

A recent internal report on corruption by the inspector general of the Interior Ministry specifically mentions the bribery of checkpoint guards: The blast on Sunday at the Justice Ministry, surrounded by checkpoints, killed nearly 160 people, while a similar attack in August on the Foreign and Finance Ministries killed at least 122.

“These car bombs didn’t come from the sky!” said Judge Abdul Sattar al-Beiriqdar, spokesman for the Higher Judicial Council. “They must have been driven in streets until they reached their target. If there were no corruption, the attackers wouldn’t risk passing through these checkpoints.”

But the corruption runs much deeper, endangering the fragile sense of security in Iraq as America draws down its forces, with security services that seem aimed as much at enriching themselves as protecting average Iraqis, according to dozens of interviews with police officers and officials as well as the report by the Interior Ministry.