Bloomberg reports that the subpoenas, issued by New York Attorney General Eric Schneiderman starting in August, bring to 16 the total number of banks that have been subpoenaed in the states’ investigation, said the person, who asked not to be identified because there wasn’t authorization to speak publicly.
Here's a list of the 16 banks thought to have been subpoenaed:
Bank of America
Bank of Tokyo Mitsubishi
Lloyds Banking Group
Royal Bank of Canada
Royal Bank of Scotland
Note that a subpoena is only a demand for information, not an indication of wrongdoing