Drawn in thick marker along the map of upstate New York, the line snaked down the Niagara River and zigzagged east to outline a swath of Buffalo and its surrounding neighborhoods.
But one area of the city — neighborhoods in east Buffalo, where more than 75 percent of the city’s African-American population lives — was explicitly excluded, cut off from access to mortgage credit.
That map, ringed by a line, is at the center of a sweeping investigation by the New York attorney general, Eric T. Schneiderman, into whether banks are “redlining” — deliberately choking off mortgage lending to predominantly minority communities.
The investigation reached its first target on Tuesday, with Mr. Schneiderman’s office taking aim at Evans Bank, a regional lender whose business in the Buffalo area dates to 1920, accusing it of denying mortgages to African-Americans regardless of their credit.
The case, accusing Evans Bank of violating the Fair Housing Act — a federal law intended to ensure equal access to credit — is a harbinger of other lawsuits that could be brought against some of the nation’s largest banks, several people briefed on the investigations said.
In the suit, filed in state court, prosecutors outlined how, since 2009, Evans Bancorp has created a map that defined the “trade area,” places in the Buffalo metropolitan region where the bank would make mortgages and other loans. The bank, prosecutors contend, deliberately excised much of Buffalo’s East Side.
Rival banks, the authorities said, lent to neighborhoods on the East Side at a far higher rate than Evans Bank, suggesting that the lending patterns did not stem from a dearth of willing minority borrowers.