The Massachusetts attorney general's office says it is investigating whether BankBoston Corp. and Fleet Financial Group conspired in setting automated teller machine surcharges.

The two banks, which control two-thirds of the ATMs in metropolitan Boston, had agreed to a voluntary price cap on ATM surcharges, then abruptly dropped the pledge in response to the attorney general's concern.

Barbara Anthony, chief of the attorney general's public protection bureau, called the withdrawal of the price cap "a step in the right direction," but said the antitrust probe is continuing.

Officials at Fleet and BankBoston denied any collusion.

Fleet announced Friday it would begin levying 75-cent fees next month; BankBoston also intends to start surcharging this fall, but has not set a price.

A bill that would have banned ATM surcharges died in the Massachusetts legislature last month.

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