Federal prosecutors and regulators on Wednesday accused two former Wall Street brokers of defrauding customers by making more than $1 billion in unauthorized purchases of securities tied to subprime mortgages.

In an indictment unsealed in federal court in Brooklyn, N.Y., the two former Credit Suisse Securities brokers were charged with deceiving customers in a bid to pump up their sales commissions. The charges against Julian Tzolov, 35, and Eric Butler, 36, were announced by U.S. Attorney Benton Campbell in Brooklyn.