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A manhunt is under way for the rest of the assailants after they fled two banks in Guararema municipality, 80 kilometers (50 miles) northeast of Brazil's biggest city, the Sao Paulo state government said in a statement.
Some of the attackers forced their way into a house where they held the occupants hostage. The hostages were later freed by military police.
Around 25 assailants were involved in the early morning raid, according to the statement.
During a search of the area police found seven rifles, four pistols, explosives and bullet-proof vests.
Brazilian TV showed one of the banks targeted by the gang was located next to a police station. The other bank was at the end of the same street.