Banks robbers made off with nearly $7 million and killed eight security guards in an overnight raid on a Baghdad Bank before dawn, the Los Angeles Times reported Tuesday.

The money had just been transferred to state-owned Al-Rafidain bank's branch in the central Baghdad shopping district of Karrada on Monday, according to the bank employee, who declined to be named.

The employee added that no doors or windows were damaged in the raid.

Police officials said the thieves used guns equipped with silencers to kill three guards outside the bank. Then they shot dead five guards who were sleeping inside the bank, before making off with the cash, according to LA Times.

The heist came as U.S. Defense Secretary Robert Gates flew to Iraq on an unannounced visit to get a firsthand look at conditions on the ground since U.S. combat troops have withdrawn from Iraq's cities.

The U.S. military said it is possible that insurgents are involved.

The biggest-ever heist in Iraq happened on January 13, 2005 when robbers made off with $13.5 million from a Rasheed Bank branch in Ramadi, around 60 miles west of Baghdad.