Eight Wall Street banks, including such big names as Morgan Stanley, Goldman Sachs and Credit Suisse, are facing allegations of misleading rating agencies about mortgage securities.
The banks have been summoned to submit documentation to New York's attorney general.
They could face criminal or civil charges if investigators decide there's enough evidence.
Wall Street giant Morgan Stanley is among those in the spotlight. Earlier reports had it that US federal prosecutors are investigating if the merchant bank misled investors to the tune of up to $400 million. In the light of the news, Morgan Stanley shares are falling in pre-market trading.