New York state's attorney general, Andrew Cuomo, ramping up his investigation of Merrill Lynch's purchase by Bank of America Corp., issued subpoenas to the five directors on the bank's audit committee at the time of the deal, according to people familiar with the situation.

Mr. Cuomo, who also is weighing possible civil fraud charges against BofA Chief Executive Kenneth Lewis, also plans to subpoena all 15 of the Charlotte, N.C., company's directors as of early December, when shareholders approved the deal.

In...