It is the second deadly robbery in a week in Baghdad's commercial Karradah district. Although violence has dropped dramatically over the past two years, the number of robberies in Iraq appears to be on the rise.
A special committee made up of Iraqi army, police and bank officials was formed to investigate the robbery, which netted 8 billion Iraqi Dinars ($6.9 million), according to an Interior Ministry official.
The gunmen broke into the state-run Rafidain Bank at about 4 a.m., killing three on-duty guards and five others on the premises, who were either on a break or asleep, said an Iraqi police official.
Investigators believe the gunmen used silencer-fitted weapons during the attack because witnesses and neighbors didn't hear any gunshots.
It was not immediately known how many gunmen were involved or how the money was secured in the bank at the time of he robbery.
Another official said the robbery appears to be the work of insurgents trying to get hold of cash to finance their operations. He did not explain what evidence led him to that conclusion.
All the officials spoke on condition of anonymity because they were not authorized to release the information to the media.
"While I cannot confirm that the attacks were terrorist related, it does fit past trends of terrorist groups in Iraq of financing their operations through crimes - like kidnappings for ransom, robberies and black marketeering," U.S. Army Maj. David Shoupe, a military spokesman, told The Associated Press in an e-mail.
Insurgents in Afghanistan also are believed to have staged bank robberies, kidnappings for ransom and other criminal activities to raise funds.
Five people were killed Sunday when gunmen broke into the al-Nibal money exchange office in Karradah shortly before noon, killing three employees and two customers and wounding 12 others, including eight employees.
In April, gunmen armed with silenced weapons killed at least seven people during a daylight heist of jewelry stores in Baghdad. In the same month, gunmen used similar tactics, killing two, during the robbery of a currency exchange office in the southern city of Basra.