New York's top prosecutor opened a new front in efforts to hold banks accountable for the financial crisis by filing a civil lawsuit against J.P. Morgan Chase & Co., alleging widespread fraud by the company's Bear Stearns unit in the sale of mortgage-backed securities.

The case is the first to be brought under the aegis of a group of federal and state prosecutors and regulators formed by President Barack Obama in January. If successful, the lawsuit could point the way to significantly more financial...