Schneiderman is investigating whether banks are "improperly denying or otherwise restricting banking access to New York consumers," the letters say.
The Federal Deposit Insurance Corp. said the number of U.S. citizens without bank accounts has jumped by more than 10 percent since 2009, The New York Times reported Friday.
Consumer advocates are concerned that bank policies discriminate against the lower wage earners, who are more apt to bounce a check or overdraw an account, which are actions banks use to turn customers away.
"Hundreds of thousands of Americans are being shut out [of the U.S. banking system] for relatively small mistakes," Jonathan Mintz, the city's commissioner for the Department of Consumer Affairs, said recently.
Banks say they use the databases to filter out customers who are high risks for bank fraud.
The attorney general's office said "African-Americans, Latinos and other minority groups," could be vulnerable to discrimination that would violate state and federal laws mandating equal access to credit, the Times said.
Schneiderman's office has sent letters to Bank of America, Citibank and JPMorgan Chase, among others, inquiring about their bank practices.