A Baghdad bank was robbed of more than $300 million in cash by its own security guards, as the high levels of corruption in the Iraqi capital once again were exposed, officials said.

In what is believed to be the biggest single theft since Saddam Hussein's fall in 2003, the private Dar Es Salaam bank was found with its front doors unlocked and its entire haul of cash missing.

Police in Baghdad said the bank's employees discovered the theft when they arrived at the premises in the city's Karrada district on Wednesday morning. An additional $174,000 had been stolen, employees reported.

Police suspect three missing security guards, who usually slept on the premises, and said they are among the only people with keys.

Police said a joint committee with local authorities had been set up to investigate the theft. In particular, it likely will investigate why the bank had so much cash on the premises.

The theft is believed to be the biggest since Saddam Hussein's fall. During the U.S.-led invasion, huge amounts of money were looted from Iraq's banks, many of which are based in Karrada, a key commercial district in Baghdad.